If your intention is to showoff to your PC buddies with games on the G5, you will be sorely disappointed. The video drivers for the PC world for the same video cards are much better/optimized. Add to that the availablity of higher-end versions of the same cards, and on a graphics' level it's not a competition point.
However, if you are doing this just to see better graphics on your system, UT2K3 is as good as any program to show some nice l&t and pixel shaders.