something is wrong?! can you help?!

2 days ago I was playing Halo2 and everytime something would blow up I would see chunks on the screen. I thought since I never really upgrade my drivers for vista that could be the problem. SO I go to sleep and I go on my computer and it would turn on but nothing would display. Im wondering if my video card went to hell? or did my PSU burnout? This is the wierd part, okay I changed my DVI cable and boom it worked.

Hours later I come home and wouldnt u know its doing the same freaking thing. SO, unplugged my fans,my other harddrive and its working right now. But, like I said t he strange thing is the computer rebooted and finally displayed and somehow it had the 3 same websites and nothing loaded. It was like nothing ever happened, like I never turned my computer off. I have no clue on what to do. Im thinking im gonna just go ahead and get a 8800,asus striker or evga 680i,and 2 more gigs of ram and 1000watt modular PSU.

What do u think the problem is?

My specs at the moment.

PSU - 500 watt antec
Harddrive- 500gb and 80gb
video card- 7800gt
ram- 2gb corsair DDR2
processor- intel core extreme quad core
case- antec 900

I was going to get a 9800 the new 9 series for xmas since the 9 series "9800" from nvidia is gonna be somewhere around x3 faster than the 8800. but thanks for the help if any!