Like the poster just before me said…186GB (200GB rounded) should be plenty of space…unless you have specific needs - where you KNOW you’ll be using more than that.
To give you an idea…
I’m not dual booting my home machine but run Vista Home Premium on a 76GB Raptor drive. Most programs (including games) are installed to this drive.
I do digital photography, and shoot raw…which means for most pics I have a 10MB version of the pic, and a 3 to 4mb version (.jpg).
I have a single folder on my system drive called “tempPics” which is where all pics go first for editing. Once they’re done, they are moved to an external drive.
I don’t know how much space OSX takes up, and you haven’t really said how much OTHER software you plan on installing.
Assuming OSX will be your main OS…and you’re only gonna use Windows for gaming…(meaning no M$ office on windows…etc.)
Looking at “worst case scenario”…
Vista Ultimate requires: 40 GB hard drive with at least 15 GB of available space
I’m guessing Crysis will be on the higher end of what you would need, and it needs 12GB
If you want only one game installed at a time: I’d go 55GB for Windows
If you want 2 games installed at one time, I’d go 65GB.
Again…these are all just guesses and estimations…depending on what you actually installed, these numbers COULD be lower or higher.
Remember though…games aren’t going to get smaller…only bigger.
And windows, will need patches over time.
As for installing games to an external drive…and making them load from there…I wouldn’t. You MIGHT be able to get away with it, IF you have a graphics card that has 768MB of memory or more…so it can load most everything there, but I still wouldn’t, since it will take forever to load up the game to start.