Depends on what you want to spend. For about $179 or so you can get a GeForce FX 5900SE, which is really good. Or for about $149 (and if you're willing to overclock) some folks are getting really incredible results with a GeForce FX 5700 non-ultra. Since you're running Linux I won't mention ATI.
Buy the best card in your price range, however make sure you keep some money set aside for the purchase of more system memory. You can buy 512 mb kits (2x256) of quality RAM for less than $100.