bilbo--baggins,
I hear you. I have a PC, on which I initially had 600GB (200GB and 400GB HDs). Then I added a NAS, which originally had 4x200GB, but I soon upgraded to 4x400GB (1100GB usable space). Now I am getting my MP fitted with 2x750GB (Raid0), 500GB, and 200GB HDs, and still all my data is on the NAS. In addition I will have a 1TB Time Capsule.
It is easy though: OS takes space, and with all the utilities etc. you've soon used 100GB. Then there are all the programs, and if you do full installs, with Adobe Master Collection, MSO, Hallmark, Corel, and various other >5GB installs, soon you're using 500GB just for applications. Then there are games, actuially let's not even talk games, but basically you're soon using 1TB just for OS, utilities, and applications.
Then of course you need a sizable scratch drive, user folders, and if you plan to run Vista through Fusion, you need an additional HD, or partition (that's 200GB just added). Finally you want to run Time Machine (which is best done on a Time Capsule anyway, but still).
And, I haven't even started talking data...