RAM, RAM, RAM...
VPC likes a good fast processor, but it LOVES a good amount of RAM...Connectix suggests 192MB installed in your Mac to run Windows 2000 or XP, but I would suggesting getting at least 392MB (really, this should be the OS X minimum...if you have less and you think your performance is good, it isnt!)
In VPC 4.0, I gave Win 2000 192MB for itself, and it ran acceptably on my iBook 500. Contrast that to my 600MHz iMac with 128 MB where 48MB are given to Win 2000 and performance is gawdawful!
Seriously, its been said a million times, but RAM is the real key to performance in OS X (and in 9, but more so in X)...all Macs sold today have at least 128, and 256 MB is only $30 (for the iBook, even less for other models).