To get a real boost from Virtual PC 6, try downloading Tinkertool, and use it to Quit the Finder whilst running Virtual PC. I find it gives a great speed boost and makes the application actually usable, even on my tired Ti 400.
[robg adds: In addition to Tinkertool, you could also use this hint to add a Quit option to the Finder's menu...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031002053525735