Maybe worth you putting both chips in and testing with
http://oca.microsoft.com/en/windiag.asp#topIts a memory test program from Microsoft to diagnose problems with memory chips. It runs from a floppy or CD.
Also worth checking the bios settings - try changing the CAS speed of the chips to 3 (it may simply be that you have different speed chips).
Alternatively to set the VCACHE etc. in ME you need to:
Run msconfig
Go to system.ini
Expand the [vcache] (there should just be an empty space below it)
Click the space below it then click the edit button
Type: maxfilecache=512000
Apply this & close down your PC
Install the extra memory & restart PC
The fix Sandra posted was to alter the amount of hard drive space used for virtual memory which is what is used AFTER the ram has been utilised.