Well, after all said and done, from my original 160 GB hard drive, I'm taking up... let's see... 160-50=90 gigs.
Wow, i'm taking up 90 gigs?! I gotta clean some crap out...
But anyway, no OS X should not take up 30 Gigs. OS X, on a standard installation, takes up about 15 gigs. Although, there's a lot of packaged stuff I don't use (e.g. Office Test Drive, I have the full version of Office), and I was able to cut my installation down to ten gigs.