So I've seen this trend for people to buy a smaller drive (less then 200gb on average it seems), stick their OS and other programs on it and then have a larger drive, 500gb+ and dump all their other files and junk on that.
I'm running a WD VelociRaptor 300gb drive, single on SATA.
I use the computer a few hours a day, haven't backed it up yet (need to, but am getting error messages via Win7 when I try to dump to a WD 250g blue drive), I use it for email, web surfing, photoshoping, playing a RC helicopter flight sim (Phoenix 3) and sometimes Gears of War 1.
Give that drives are pretty darn cheap, what advantage would I really realize by sticking in some smaller drive and having my OS/programs migrated to it and using the raptor to hold my storage stuff?
You would (could) see slightly faster load times. Another alternative would be to have your OS/favorite programs installed on a smaller partition on the main HDD. You may be able to shrink the volume that your OS is on thru disk manager. This simply reduces the amount of data that needs to be searched to find your OS/ programs.