Just some quick comparisons of benchmarks of my old machine (Athlon 800) and my new one:

Benchmark Increase
CPU int (dhrystone) 2.83x
CPU float (whetstone) 3.38x
CPU 3DNow! 2.49x
RAM int bandwidth 3.95x
RAM float bandwidth 4.05x
Cache and RAM "combined index" 3.77x
3DMark 2001 ~4.75x

I certainly don't think I have the most beastly machine in the world (Nomad has more memory and a pair of RAIDed SATA drives, for instance, and at least one person I know at work has a Radeon X800), but I'm amazed with how much faster this machine is than my old one. I guess that's what happens when you wait 5 years between upgrades.

Update 08-28: 3DMark 2003 benchmark for the new machine: 3811 3DMarks.
Update 08-30: Doom3 High Quality timedemo: 30.4 fps (I couldn't run Ultra Quality because I only have a half-gig of memory)