Yeah, I think 350 watt power supply isn't nearly enough for the radeon 9800. I have a radeon 9700 w/450 watt power supply, and I did have to make some initial changes, as some of the hard drives wouldn't power up correctly and such. Of course, I'm using a server case, with like 5 case fans, a window/light mod, 3 hard drives, floppy, 1 cd-rom/writer, 1 dvd-rom/writer, as well as some other various usb and pci devices.
When I unhooked the light mod and 1 fan, and took out the tv tuner, it gave me just enough juice to run the video card with no problems. Best bet is to get about a 700 watt one, and you'll have a bit of upgradability left on the PC. Otherwise, you might be limited to what you can add, as far as fans, pci cards, more drives, etc.