Ok, how many other drives do you have?
Modern motherboards generally come with 2 IDE controllers. Each controller can handle 2 IDE devices, for a total of 4 IDE devices.
You have 2 hard drives, and almost certainly an IDE CDROM drive. If that's all, then you have a controller available. You can just drop in a new drive & plug things in. Make sure to get the master/slave jumpers set up right.
-<i>but</i>-
If you also have an internal Zip drive or CD burner, you've used up your last onboard IDE spot (unless they are SCSI, which is unlikely). In this case, you'll have to get a PCI IDE controller card as others have mentioned. Another thing to look at is if you have a RAID MB which would give you 2 more onboard controllers. But you would probably know if you did.
As for the subject question...
You can get PCI IDE controllers that can handle up to 8 drives. If you have 4 free PCI slots, that's 32 drives plus 4 on the onboard controllers and 2 more on the onboard RAID controllers for a total of 38 drives. Hang an 80 GB drive on each of those and you have 3 Terabytes!
Of course you'll need a hell of a power supply
In theory, there is no difference between theory and practice.
In practice, there is.