Hrm, didnt seem to let me post it all, reposting top bit:
"I've recently built a new system, the spec which i will post below this, and i've come across a bit of a problem. I am able to run Crysis well on full settings, Sims 3, Bioshock, World in Conflict etc. all run near flawlessly on full with 1920 x 1200 settings. However, for some reason i'm coming across a strange problem with the whole system freezing when playing Sims 3 or Crysis.
At first, i believed only Crysis was the problem. It was the only game that had crashed in this manner throughout my PC's short life-span. That was of course until today. I've installed Sims 3, and was able to play it for about 2 hours before the very same crash took place, with the same symptoms.
Crash Symptoms: The crash can take place at any point (in Crysis) from about 5 to 60 minutes of play, the most common timeplace being about 20 minutes. The game will be running well (averaging 30 fps, full settings, no AA, 1280 x 1024 [i've gotten a new monitor and uninstalled crysis recently]) until the crash begins. At first, there will be a noise coming from within the pc, as if something inside were shutting off, or a fan had stopped turning or something. After a second of the game continuing to run, the image will either freeze in place, or the display will simply disapear, leaving my monitor telling me there was no signal. Both of these two symptoms are accompanied by repeating the noise of the last half-second of life of the game. At that point, ctrl-alt-del does not work, the system is completely seized up. The only thing to work is killing the pc's power.
Extra Info: Sometimes, i will hear the noise inside the PC, and -if i'm lucky- the crash will be put off a few minutes, which is what happenned the first time in the Sims 3 crash. In the Sims 3, the crash took place after about 2 hours of play the first time, but the second time, in happenned within about 50 minutes. I think that the crashes are happening at graphically intense times, which would explain why only Sims 3 and Crysis are effected (though World in Conflict is by no means poor graphics, which makes me doubt this) as when playing the last level of Crysis (where the giant alien walker attacks the ship) the game was crashing every 5 minutes or so. However, in Sims 3, the crash hasn't happenned enough to warrant a decent guess towards whether this hypothesis is true.
Not sure if it's overheating, the CPU is kept relatively cool, bordering the 60c barrier. THe GPU can run a little hot, in the early 70s, but i believe the cut off point is about 30 degrees hotter."