What operating system, that part is important. Go to the Apple menu, About This Mac. It will tell what version there.

If it's OS X then click on More Info, what is next to Model Identifier, probably something like PowerMac 5,1, is it one of the old towers made out of plastic that has the door on the side with the round latch that drops down? That is important also.

I take it you meant 1 or 2 terabyte drive? How much data is on the hard drive? Click on the hard drive and go up to File/Get Info.