What good is it to overclock if you can't brag about how fast you got it going?

3DMark makes a very nice shareware Direct3D utility called 3DMark2000. It allows you to score the performance of your card and your entire system. You can also upload your scores to a worldwide database so you can show them off.

Really fun stuff for tweakers

You can benchmark your system with games that you may already own. "Quake 3" is probably the most widely used OpenGL benchmark in the world, and nV News has a great little guide that explains how to benchmark with specific games.