My 9800 GX2 had been maxing at something like 110 C, so I decided that, after four years, it was time to replace the thermal compound. I ordered some MX-4, applied it without a hitch, and put it back in my PC. I started up Red Orchestra 2, and after a few minutes of play, the temperature did not exceed 80 C. The game then froze and sputtered as if it were about to crash, then came back to life and played normally. It did this again a few minutes later. A few seconds after that, it did it again, then my system BSOD'd.
I started up my PC again and let it run overnight, and when I came back the next morning it was still running fine. The next day (the day of this writing) was moving day for me, so I packed up my PC and drove it to my new apartment. Double-checking that everything was in place (including the PSU connection to the card), I turned my PC on. It took a try or two to get anything to display, but after jiggling the monitor cable, it worked fine.
Then I noticed that my PC was running a tad slower than it had been before (all around). I checked my task manager, and CPU levels were spiking but mostly normal (so I'm not entirely sure this isn't a CPU problem - my CPU is idling in around 30-35 C). I noticed Source SDK was running a bit slow as well, but no big deal.
Then I started up Red Orchestra 2 again, and before I even got to the menu, it was like watching a slideshow. I think it has something to do with my card, though I'm not sure. It ran just fine before it crashed last night, so I'm hoping one of the cores didn't fry (it's a dual GPU, but both cores appear to be running in the Nvidia panel and on SpeedFan - as for the crash, it could have been something to do with the fact that I changed from a cable to wireless right before playing, so I don't know).
EDIT: Forgot to post specs:
Windows Vista Ultimate 64-bit
AMD Phenom II x4 970 Black edition (four cores, 3.5 GHz)
Nvidia 9800 GX2
MSI K9N2 Platinum Edition motherboard
750-800W PSU (I don't recall)