I'd definitely get a 350w power supply, even if it's a cheapy (they mean 350w cheapy when they say 350w, anyway). My old GeForce FX 5900 Ultra required 350w PSU and it ran fine on my shitcan POWMAX PSU 350w.
Just order a standard ATX PSU from an Internet site (I suggest
www.newegg.com ,
www.directron.com ,
www.xoxide.com ). Xoxide actually has 350w PSU's on sale sometimes for $8.
Expect to pay about $30 for a 350w cheapy at a computer store.
If you want to get a really nice PSU (they'll handle way more components, are more reliable, and MUCH quieter) then I suggest Enernax, Sparkle, Antec, or SilenX. Expect to pay about $50 for a 350w for the aformentioned at a website, and closer to $70 in a computer store, perhaps even $100.
The bottom line is: if you have 1-2 hard drives, 1-2 CD-ROMs, and the x800 XT Platinum Edition, a POWMAX or RAIDMAX cheap PSU should do fine. I personally believe buying an expensive, quality PSU is worth it, but the videocard should work on a cheap PSU.
Changing out the PSU in your computer is really, really simple. Just remember where everything is plugged in, unplug it, unscrew the screws on the back of your case for the PSU, take it out, install the new one, and plug it back up like you had it before.
-Lasereth