If you want it to be MacOS compatible (and not just Windows compatible), the best card right now is the Mac version of the ATI Radeon X1900XT for gaming. The expensive Nvidia Quadro 4500 is probably the best for 3D workstation type applications. Unless you're working in CAD, Maya, Cinema 4D, or some such app, you probably want the ATI Radeon X1900XT.