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The other thing to consider is the speed of the drives. If you plan on just using the drives for storage get somthing like western digital Green drive. Either 1tb or 2tb depending what ypou can afford. Like said above a good way to go could be to get 2 1TB drives and have them in Raid 1 to protect yourself if there is a problem with 1 of the drives. If you plan on working directly off the drive. It becomes harder, you want at least a 10,000 spped drive to work off so maybe look for a 360Gb Western digital velocoraptor and then just get a slower 1Tb drive for storage.
However you do need to know how many drives you can instal. Full tower desktops obviously have more room but even mid towers can normally fit 5 drives so you should be good to go unless you already have 3 or more drives.
Regarding 3TB drives they often require an update for the motherboard to be read correctly so you may be best off sticking to 2TB drives