How to determine type of internal hard drive for replacement?

I have a hand-me-down computer that had its inteal hard drive removed before it was given to me. I'd like to buy a new HDD and install some flavor of Linux onto it. Is there a diagnostic utility that I can, say, bu to a CD, that will tell me the specifications I'll need to know when buying the new HDD? Or, is there a way to get all of the required info from poking around in the machine's BIOS?

Put another way, what details do I have to know to find a HDD that will work when installed into the machine?