I have been working with MS Virtual Server 2005 a lot recently. I must say, overall it is a nice and efficient solution. Although it has some limitations that do not exist in VMWare, for the overall cost it is a good solution.
I am running five servers on one server. The box has a few IDE hard drives, 4 gig of memory and a 2.8 ghz processor. Aside from the memory, it is not an impressive box, but it runs all the servers with surprisingly high performance.