It's very difficult to predict the performance of VMs, but you should look at benchmarks that use disk I/O like AVG Anti-Virus, Linux GZIP, Linux Kernel Compilation, WinRAR, etc. You can safely ignore all benchmarks that measure CPU, GPU or memory performance only as they are not relevant to a VM server.
An Intel or AMD Quad that costs ~$200 should meet your requirements. The CPU The best CPU for VMs is the Intel Xeon 7400 Series, but building a system based on that CPU is very expensive.
You'll probably find out that your stripe of 80gb 7200rpm hard drives is a bottleneck with a very fast server. In a test environment, one fast SATA disk per VM, e.g., a WD6400AAKS, works quite well and it isn't that expensive. Using them in a RAID configuration (4 disks or more) to run several VMs should also be a good solution.