I'm aware of what 64 bit means. It's now extended up through the entire OS though, and not just at the UNIX layer like it was in Tiger. Regardless, the performance has dramatically increased when compared with Tiger, which isn't slow at all itself.
Each OS X release seems to consistently perform better on the same hardware, even though more features have been added. I'm obviously talking about a reasonably recent system. It's obvious that a G3 (which isn't even technically supported by Leopard) isn't going to increase in terms of performance. Within reason each OS X release has always increased performance over prior releases.