i have a 750 Gig external, i download movies and music for me and my family, I've had it for 2 months now and I've yet to use up 100Gigs
if your planning on saving ALOT of games,movies and videos, then go for it, by the way by alot i mean ALOT, i have full seasons of shows like family guy, it's awsome since i'm running a media sever but again, i haven't even used up a fourth of it, i doubt i'll ever max out a 750gig