Well... lets see
Final Cut Pro takes up approx 1 gig
SoundTrack takes up about 5gigs
LiveType takes up 9gigs
Motion takes up 10 gigs
and DVD Studio Pro needs 20gig's, in terms of both the Preset Templates, and space for rendering final DVD Project.
so you do the math 😛
15gb's of free space is needed for the installation of FCP, Soundtrack, and LiveType, add Motion and that number jumps to 25gb's... then add DVD Studio Pro, with all the tutorials, presets...etc, and you bring that number close to 40gb. you could save some space by not installing the supplemental FCP discs, or you could possibly save the files to an external (I believe you can do that)
(all info from apple.com)