I chose the recently free'd Citrix XenServer and I am mightily impressed. It just wipes VMware off the floor when it comes to I/O performance due to para-virtualized drivers. The NIC drivers are also fast and efficient. I solved the shared disk problem by making one VM an iSCSI target. I can present as many LUN's as I want to other VM's now. since all traffic goes over the internal NIC, performance is excellent.
The XenServer product also consumes far fewer system resources than VMWare and the quality and polish is really good. not to mention the excellent documentation that is available from Citrix
You can save a lot of money by choosing AMD based chipsets and a cheap Phenom or Athlon X2. CPU clock speed may not buy you as much as fast disks and RAM.