Processing power required for multiple virtualized dev servers?

We have about 6 Dell power edge servers used by my Dev team ranging for SQL instances to automated build servers.

I tried to take one of these 2x2.3GHZ with 1 GB of memory and ran MS virtual server 2005 over it. Now when I finally got windows 2008 开发者_StackOverflowinstalled, it ran, but it was slow!!! The CPU seems to be bouncing off 100% alot of the time. ( Bear in mind it was a base install with nothing else turned on )

With this in mind I'm thinking of turning to VMware ( Installing it as a bare metal install i.e. its own OS )

But my question is, if I wanna run 5-6 instances of windows 2008 ( All will be low activity instances apart from I'll add an exchange server for 10 people on top of this )

what processing power and memory would be a good starting point when adding a purchasing a new machine for this. I dont want to buy something and end up in the same situation where the clients run like dogs.

Ok I finally got round to this and installed the free VMWare ESXI barebones to compare. Performance is so much better! ( Not sure why MS was so sluggish )

J