Virtualization is definitely the answer here. If you want multiple OSes at once, you have to go the virtualization route. It would be very time consuming to have to continually ghost, or even reboot a machine to boot into a new OS.
The only drawback I see is machine resources. Get a big machine with lots of memory if you plan on using several virtual OSes. VMWare Server is a nice product that will allow you to setup the VMs and allow users to access them remotely as well (and its free!).
Virtualization. If the one machine has enough horsepower/muscle (RAM/CPU/Disk) you can run those multiple machines at the same time thus besting the ghosting option.
In your situation I'd go with virtualization. It gives you the ability to run multiple images on one physical machine, and also to spin up new instances as needed. This can be very useful for testing.
There's no reason not to do both. If your application isn't overly hardware-needy you can virtualize, but a spare hard drive can give you the occasional bare-metal testing -- your guest VM images can be ghosted over to the spare drive, reboot, and voila.