I have an ATI Radeon 9800 Pro on and Abit nForce2 motherbaord with an AthlonXP 2800+. It has worked flawlessly for me so far in everygame I have thrown at it, including Doom 3, Halo, Far Cry, MS Flight SIm 2004 and now Half Life 2.
nVidia based cards probably due run a little faster on nVidia based motherboards, but I haven't seen any evidence yet that it makes a noticeable difference.
For a budget, the 9800 Pros are probably the best value for your money. The new GeForce 6x00 and Radeon Xx00 are much faster, but cost more.
goates