One of the main reasons that XP doesnt like the hard drive moving from one pc to another, or new mobo/cpu as you are doing, is that the IDE controller drivers arent installed so it cant detect the boot sector of your hard drive.
If the new mobo happens to use the same drivers then you are usually ok.
To have a better chance of it booting on a new setup is to install the extra IDE controller drivers that XP can use.
You can add a small registry file to the registry that includes the extra drivers.
Copy and paste all of the following text in the quoted part to a notepad :
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\primary_ide_channel]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="atapi"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\secondary_ide_channel]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="atapi"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\*pnp0600]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="atapi"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\*azt0502]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="atapi"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\gendisk]
"ClassGUID"="{4D36E967-E325-11CE-BFC1-08002BE10318}"
"Service"="disk"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#cc_0101]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_0e11&dev_ae33]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1039&dev_0601]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase \pci#ven_1039&dev_5513]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1042&dev_1000]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_105a&dev_4d33]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0640]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0646]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0646&REV_05]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0646&REV_07]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0648]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1095&dev_0649]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1097&dev_0038]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_10ad&dev_0001]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_10ad&dev_0150]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_10b9&dev_5215]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_10b9&dev_5219]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_10b9&dev_5229]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="pciide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_1106&dev_0571]
"Service"="pciide"
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_1222]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_1230]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_2411]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_2421]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_7010]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_7111]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\pci#ven_8086&dev_7199]
"ClassGUID"="{4D36E96A-E325-11CE-BFC1-08002BE10318}"
"Service"="intelide"
;Add driver for Atapi (requires Atapi.sys in Drivers directory)
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\atapi]
"ErrorControl"=dword:00000001
"Group"="SCSI miniport"
"Start"=dword:00000000
"Tag"=dword:00000019
"Type"=dword:00000001
"DisplayName"="Standard IDE/ESDI Hard Disk Controller"
"ImagePath"=hex(2):53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,44,00,\
52,00,49,00,56,00,45,00,52,00,53,00,5c,00,61,00,74,00,61,00,70,00,69,00,2e,\
00,73,00,79,00,73,00,00,00
;Add driver for intelide (requires intelide.sys in drivers directory)
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IntelIde]
"ErrorControl"=dword:00000001
"Group"="System Bus Extender"
"Start"=dword:00000000
"Tag"=dword:00000004
"Type"=dword:00000001
"ImagePath"=hex(2):53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,44,00,\
52,00,49,00,56,00,45,00,52,00,53,00,5c,00,69,00,6e,00,74,00,65,00,6c,00,69,\
00,64,00,65,00,2e,00,73,00,79,00,73,00,00,00
;Add driver for Pciide (requires Pciide.sys and Pciidex.sys in Drivers directory)
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PCIIde]
"ErrorControl"=dword:00000001
"Group"="System Bus Extender"
"Start"=dword:00000000
"Tag"=dword:00000003
"Type"=dword:00000001
"ImagePath"=hex(2):53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,44,00,\
52,00,49,00,56,00,45,00,52,00,53,00,5c,00,70,00,63,00,69,00,69,00,64,00,65,\
00,2e,00,73,00,79,00,73,00,00,00
Save it as MERGEIDE.REG
Then double left click on it to add it to the registry, say yes when it asks if you want to add it to the registry.
Hopefully if you move it to another mobo it will boot ok.
Re the ram :
The motherboard supports dual memory but can also run as a single stick or two sticks of different size ram in single channel mode.
If you do have two identical sticks of ram then it will run in dual channel mode which is more efficient.
If you are struggling to get a 939 cpu then have a look on ebay as someone may have upgraded their mobo and cpu and have a cpu for sale.