I can't speak for Windows 7 or 8 with authority although I do use W7. There are some cases in Windows when you can drag an app to a new location and it will work. In other cases it will break, I believe this is due to where the registry has components recorded. This definitely was an issue in XP, but maybe with newer Windows versions they have overcome this problem although W7 still has a registry. Anyone know if W8 has one?
I'm sure you've noticed that all hard drives appear on the desktop. The MacOS in contrast is very forgiving in this regard (dragging apps around). Any game you have installed, you can drag it (the game folder) to an external HD and it will function just fine. Remember to erase content in the old location, but it's not required, just to save space. And new games can be installed directly to the external drive, no issue at all.
With my MBP, I have a 1TB drive I travel with. I have it divided into two 500GB partitions, using one side for file storage, extra content/games and the other side for Time Machine, which I strongly recommend you take advantage of.