Random freezing and Memory Management BSOD

OK, so I recently had to replace the motherboard on my machine, and then had to take it to the shop when I was completely stumped by a mysterious problem that was preventing it from turning on -- it turned out to be a burned-out power switch, of all things.

That got it up and running again, and it works fine for basic things like word processing, the Internet, etc. But every time I try to play a game (any game from Crysis to Sims 3) the machine will either a) Freeze up within a couple of minutes, with no error message or anything, or b) Close and give a blue screen with a Memory Management error. These are all programs that I was able to run with room to spare before the mobo incident, and no other parts have changed.

I really have no idea what the specific problem is, other than somehow my machine doesn't think its memory sticks are there. Could one of them be burned out? Any suggestions?

Intel Q9950 2.90 gHz
Intel DP45SG mobo
4 GB Patriot DDR3 7-7-7 (2x2GB)
Radeon HD4870
NCTZ 850MW power supply