Well my friend it depend totally on your computer`s capacity.
If you have Pentium 3 Processor then you can use maximum 80 GB hard on P3 Processor.
But if you have P4,
Dual core and any other processors then you can add more than 1 TB hard dish with it.
Many hard drives are available in many capacities and cheap prices.
I think there are many capacities hard drives are available in the market.
Many varieties of hard drives are available in the market
You can add maximum amount of hard drives in your computer that can supported well
without having any problem with it.