I'll also assume you are still running XP unless you upgraded.
but you do have the Dell 375w which is solid can peak above 400w
the ATI 4770 or 4850 will work well, both where under $100 last I looked. If you want to go the Nvidia route you have the 9800GT but the ATI would offer you better performance for your dollar. No need to spend more then $100 for your video card.