Required partition size depends greatly on the OS you plan to install. For XP and AH + couple other games, 30 gb is enough. For Vista/7 even 40gb is going to fill up very fast so I'd make it 60. You can try to make vista/7 smaller by disabling volume shadow copy, system restore, deleting backup system files after updates etc. but it's far easyer just to reserve enough space to start with.