i was curious about this too as i used to use my pc basically just for gaming and wanted to know if i could continue with my new macbook pro. I asked about it to this guy i know at geek squad and he said that you cannot install things on external hard drives. Ive been able to install a small game onto a 2gig flash drive but nothing other that. what you can do is install the Windows OS of your choice with the minimum amount of space to hold only the OS and one game and some extra space for speed. then when you want to install new games transfer all the game files onto you external hard drive when you arent using the game.
so basically don't uninstall the games, but move their files onto you external hard drive for storage and bring them back to your internal hard drive when you want to play.
since game files are big i would suggest getting a external hard drive that has at least 7200rpm and has a firewire connection because they are faster than USB and Macs are built to use them.
I have an Iomega UltraMax 750GB external hard drive that has a firewire 800 connection that I can connect to my MacBook Pro