Update :

OK, well the day after I started the thread, I started up the PC and everything was working fine :S

It worked for 2 days, then all of a sudden the screen went out.

The next day I turned the PC on and it was working again.

My guess is that my PC doesn't like being on for more than a few hours. I don't think it's overheating. My CPU is running at 40 degrees, and my GFX card is at 60 (I improved the airflow in the case - it was running at 80 degrees for 2 years). Everything else is around 40.

Also, when my screen went out, I was playing COD, and the sound was still running; however after a few seconds, the game crashed. Does this suggest that the GFX card is buggered or could it still be the motherboard?

Im still going to say bad capacitors my Hp with the socket 478 did the exact same thing-literally. at first it would run for awhile then the screen would go blank. I had an ati 1250 video card in it, 256 meg of ram, and a 250 watt PSU, and a 40 gig hard drive, all of which I have used in other systems with no problem. the only thing I found with an issue was the leaking capacitors by the socket. the other HP i worked on, all the parts worked in other systems, but the motherboard did the same thing with all other parts. there was no outward sign of bad capacitors on it, but it was obviously the motherboard. my money is on your motherboard being the issue.