I installed FC5 on a test box at home, doing a fresh install over a previous FC4 install. Then I followed this guide to install Xen, it really was as easy as the guide suggests 
So now I have a second instance of FC5 running on the same box, it is pretty much as fast as the host install and you access it via ssh/VNC. Soon I will have a crack at updating my dual xeon server, you can fix guest os’s to particular cpu’s so in theory I should be able to have the parent os running on one logical cpu (the xeons are HT) and then up to three (or more, you can assign more than one guest per logical cpu) guest os’s to maximise resource utilisation.
If you have considered taking a look at Xen I strongly recommend an install of FC5 and then installing Xen via Yum, it’s not going to improve your crunching power but you will be able to consolidate some of your file/print/web servers to a single box.