Video gives out, but computer still on

Hi!

I am currently using a desktop computer with the following configuration:

Intel Core 2 Quad Processor Q6600 8MB L2 Cache 2.40Ghz 1066Mhz
Asus P5KPL-CM Motherboard
KINGSTON 4GB (2X2GB) DDR2-800 CL5 (KVR800D2N5K2/4G)
Onboard Audio
Western Digital 320GB SATA II 7200RPM 16MB(WD3200AAKS) + a 250Gb drive (forgot type, I think its Seagate)
LG 22X DVD RW w/ Lightscribe +- Dual Layer Serial ATA
Antec Sonata Family Sonata III 500 PIano Black Quiet ATX Mid Tower 500W 80-Plus
WinXP SP 3

I have added an extra fan to my case, so that the air can flow from the back to the front flawlessly. For some reason (although this is besides the main problem), but sytem fan runs at about 900RPM, while my other fans are over 1900RPM...I can't seem to boost the speed to my system fan (no control box either for "lo-mid-hi" speed). The temp for my system runs around 32C all around, the CPU sometimes hits 50C, which my temp program shows as hot.

So I figured that the reason my computer would crash would be because of the heat, but after letting it sit for 2 days, with the computer off, it shut down not long after I opened it (maybe 1 hour). Well, the computer wouldn't shut down, I would lose video signal...but the computer would still be running, lights, fans and all (although the programs are crashed, since in one instance, on WoW, I apparently DCed when my computer crashed in this way). This is a fairly new computer, as I purchased it barebones back in Sept 08. I added the extra hard drive and video card (ATI HD 2600XT 512Mb). I also have 2 USB hard drives connected and powered on all the time. It has worked flawlessly since I got it...until a couple of days ago when it started crashing.

I was thinking that maybe it was the heat, so I pointed a fan to it these last few days. Then, I thought it could be that I don't have enough power from my power supply, or that my video card was failing (which is what my instincts say). I just wanted to ask you guys your opinions without me having to purchase a new Power Supply and/or Video Card.

Any ideas?

Thanks!