Before we launched the beta of the enhanced client, we tested the client on a large number of graphics cards. Because we are using a 3d engine and hardware to render a 2d game, there is a very large amount of texture data that needs to be sent to and stored on the graphics card. It has gotten better since KR (you may remember people saying that KR made their graphics cards smoke) but it is still the bottleneck for most systems. This means that systems with large amounts of video RAM and high texture throughput will perform the best. Texture throughput is basically limited by bus speed. Older (especially onboard) video cards are notorious for have really slow texture throughputs. The newer onboard intel graphics chips have gotten much better recently.
Ok I'm done rambling now, I just thought this was an interesting bit of information.