Gigabyte 6600GT Heatpipe problems, I think...

Right, just got my new gfx card, as in the title, except every so often my pc will just restart its self. I'm starting to wonder if its a PSU problem, mines only a 300W, although if that was the case would my system even turn on? The thing is its not consistant where it restarts. It's now impossible to use Divx or media player as my entire system just goes into slow-mo. Any ideas?

The entire setup is :
Windows XP, XP 1600+, 768MB RAM(PC2100), 30GB Maxtor HD(one of the older ones), 250GB Dimond max 10, creative PCI 128 sound card, DVD ROM drive, all stuck to and Asus A7V8X-X.

Thanks for any ideas.

Your PC would turn on with a dodgy PSU but your PC won't be very stable. A good quality 300W should be enough for what you are using but if it's got weak rails then it's gonna be a problem.