I am torn between nVidia and ATI. ATI make better hardware, BUT the drivers for nVidia cards are usually much better (try to find a driver for ATI cards that will work well with linux).
Having said that, I currently use an ATI 850 XL, slightly overclocked. My system is not far off yours, (I have an Asus mobo but the rest matches) So far The 850 kicks but with games (Eats up DOOM 3, Far Cry, etc., etc.) and handles all of the graphics work I throw at it. I bought it about three months ago, for well under $200 from newegg.
