It's on the first page when you plug your phone into iTunes... you've got options to:
Manually Manage or
Only sync checked songs
I can't remember the exact wording because I'm away from my computer right now but those are the basic options when you plug the phone in.
its a lot, but i have friends with WAY more than that, i have 2 buddies who have almost a terabyte between them. they basically collect music. when they hear somethign new that they like, they find anything they can by said artist. its pretty intense.....