From experience here, computationally intensive programs won't see much noticeable difference between VM platforms, if you're running them on a modern CPU with virtualization extensions turned on.
The disc intensive things are the big part. More or less, everything you're thinking about (VirtualBox, VMWare, VirtualPC, etc) will have slower disc performance than the host system, because it's a filesystem in a file, on a filesystem. If you run multiple VMs at once, it gets crappier, especially if they're IO-intensive.
I'll suggest two ways to get around that:
1) The way to get around this is to either use some sort of dedicated external storage device. In industry, it would be a iscsi SAN or huge NAS. For home usage, use a spare box (raid helps) and set it up as a NAS. Then mount it in the VMs, and do the disc-intensive work on the network drive. It is often faster using NFS or SMB for intensive IO than accessing a local-virtual-disk, even if the NAS is your old p4 with software raid.
2) If you only have one box, but have spare internal or external hard drives, use a different physical drive to hold each virtual drive (hackish, but it really does help when running multiple.
Not a direct answer to your question, but hope one of those helps.
(wrote this yesterday, forgot to hit submit)