I would keep Mulder, ARod, Beltran, Boone, Gibbons, Loaiza, Buehrle.
It may come down to 2 of Koskie, Loaiza, Gibbons. Koskie is damn good for a 3b in the american league, assuming he's healthy.
I'd take Gibbons over Koskie. Gibbons is a great young player and only going to get better next year, plus he can play alotta positions. Koskie may get injured and is a pretty average third baseman. IMO Loaiza will put up something like 14-9, 3.70 ERA next year so I'd keep him around.