increase performance of virtual machines on the 27" imac
I have found that running VMs from a USB drive seriously hurts performance because of the bottleneck of the USB drive. I recently set up a USB drive and a Firewire drive in a RAID0 array and running Virtual Machines off that is far better than running them off just the USB drive was.
For improved performance, get a NAS device. The iMac has 1000BaseT ethernet so running VMs off a network share is also relatively fast. You can even make your own cheap NAS device with linux and a RAID array.