0x50 - basically data was requested from memory, and it wasn't found... or what was found was incorrect and had a bad effect on the pc.
Ok, well TEST YOUR MEMORY! - if you have 2 sticks, pull one out. Test until BSOD/other problems - then swap with the pulled out stick with the one thats in there.
If you still get errors, then I would probably suggest that the mainboard is faulty - with the problems with the graphics, I would suggest that the NORTHBRIDGE CHIPSET is damaged in some way - this is the chip usually found below the CPU, and will normally (these days) have a heatsink on it (and perhaps a small fan).
This NBC is like traffics lights for data flowing between CPU, MEMORY, AGP and SOUTHBRIDGE CHIPSET (another story), so if this goes faulty then replace mainboard - as you can tell from the heatsink, it does get stressed.
Good luck with that memory testing! I hope its a stick of memory because that will probably be cheaper to replace (mainboards are quite reasonable now tho)...