video intensive programs crashing after half hour

Well, I just built my computer here... not to long ago. Been crashing on video intensive programs (read: Games) since I built it. Also crashed running other programs, but that was a faulty ram stick which is getting replaced by corsair right now. But, onto the specifics.

After about a half hour or so. Sometimes as little as five minutes. Once in a while upto a half hour, the program crashes and gives the windows "report error" box, and occasionally it blues screens. It gives a variety of blue screens "0x00000050 (0xE404D96, 0x00000001, 0x8054a168, 0x00000000) is and 0x00000050 (0xA52A4A08, 0x00000008, 0xA52A4A08, 0x00000000). The errors from the event viewer are (note: these are all from portal the new game from valve)
"Faulting application hl2.exe, version 0.0.0.0, faulting module unknown, version 0.0.0.0, fault address 0x3e5e697e."
"Faulting application hl2.exe, version 0.0.0.0, faulting module engine.dll, version 0.0.0.0, fault address 0x00255350."
are a couple of the errors.

I've tried a few things, I've switched out my video card from an 8800GTS to a spare X1600 my friend had. I've moved my ram around in the DIMM's. I've tried formatting and reinstalling windows, but it's still crashing, which leads me to think it's a hardware problem.

My Hardware:
N650SLI-DS4 motherboard
OZC 750 watt power supply
8800GTS evga videocard, now an X1600 ATI
500 Gig seagate barracuda harddrive
LG dvd burner; which is having it's own problems. I'll restart my computer and see what the bios is calling it. In the bios it says "MATSHITARV9 DOWNLOAD"
X-blade case.
and a floppy drive I found in one of my old computers. And yes, my computer was crashing before I put that on.

 

It sounds like you may have a cooling problem, video card may be overheating and shuts down, not enough airflow over the mobo.