Depends. Once you get involved in torrenting that will fill up quick. Id go with a 320 GB (Or more) Seagate drive with a 16MB Cache. Should last awhile. WOuld also be a good idea to get another similar harddrive as external for data back ups..
If you don't plan on torrenting or P2Ping then 80 or 120 GB's should be enough.
I have that WD 250GB you chose, and I'm torrenting, but haven't filled it half way....still plenty of space...well a 320GB isn't much more. It's only about $20 more for 70GB more space.