100 gb imo. The difference between the 5400 and 7200 isnt really that noticeable unless you do heavy video editing work. 60gb will poof before you know it, and after applications, restore partition (if you keep it), windows xp, system restore, hibernation space, etc., you'll probably have less than 40 gigs free if you chose the 60gb drive. Not to mention that many games nowadays take up 4gb+ space! Wont be able to do much video editing with what's left of that
Edit: Didnt see the part where you said you were attaching external drives. My point still stands though. It really depends on how much video work you do and how serious it is. It wont make a huge difference for gaming.