Falleron, VPC loves memory, especially if you are running windows 2000 under it. I have two Macs, an iMac Flower Power, and a new iBook 600. I have 1GB of ram in the iMac, and 384 in the iBook. If you go with the 768, give the virtual machine 512 MB, and you will see a big difference. In fact it is Altivec aware, and supports dual processors, so you should have a zippy Virtual Machine.
Mine are fast, and I am on a G3 600 both on the iMac and the iBook. But to answer your question, yes, you will see a difference.