I ran two 250gb ide dirives and two 500gb satas in the last build. That saw XP Home on the first ide, Vista, Mandrake, Solaris 10 on the second ide, and XP Pro on the first sata. The second sata was simply a storage and backup drive put into a new case.
You would surprised to see how much drive space can be taken up between updates, iiso images for Linux, Video captures, screen shots in jpg format, and a good number of large files. The updates are for more then one OS just to start with.
The main problem after a visit to Western Digital is finding any 1tb model? The largest seen to date is their 750gb sata drives. Have a link for the 1tb drive you saw? Or were you going by the model number?