How are you determining the speed? I mean, as far as a human is concerned we have almost no chance of detecting the difference between the two systems if you're just futzing around in windows, mostly since the only delay you'll see is when it accesses the HDD, which will be almost the same speed in both systems.
If you're comparing performance in 3D games it is well known that even with a PIII 600 and a GeForce2, the video card will still be the bottleneck. So don't expect any performance increase while using that TNT2 card.
If you want to see how much faster your system really is running Seti@Home and compare times to finish a work unit.