In my MacPro, 500GB in bay 1, 300GB in bay 2 and 400GB in Bay 3 as well as a Fantom Drives 500GB USB 2.0 external (MacMall is just down the street from me and they had them for $200 with a rebate).
The 300GB is dedicated entirely to iMovie projects, the 400GB is for FrontRow (though I'm using iTheater for right now), the 500GB is for games, frontow and because I thought the 400GB was dying. LOL The external enclosure I had it in didn't like it apparently.
The 500GB is just for backup purposes now (I was using a MacMini before).