Turns out LaCie makes an excellent portable FireWire bus-powered 7200 rpm hard disk which implements a RAID 0 solution. The LaCie Little Big Disk Quadra does it all. This, my friends, is the ultimate drive for running a VM from an external drive to a MacBook Pro. It's 7200 rpm, RAID 0, FireWire 800 and has 400 GB of total storage space. Perfect.
I recently decided to switch to VMware on my Mac for hosting my Windows installation. I started off with the VM disk image on my MacBook hard disk. I have a MacBook Pro with a 7200 rpm 200 GB hard disk. The performance of the VM was not ideal. Obviously there is disk contention between Mac OS and VWware. But I hesitated to move the file over to an external drive because I didn't feel an external disk would perform any better. But I went ahead and put it to the test.