PC Requirements

Hey all, I have a question...is there a minimum requirement in mghz speed on a computer that will run my project cnc router? I have several old pc's laying around one being a pentium III 800mghz. I think I read somewhere that the speed of the computer has alot to do with the steps on motors? I really dont want my home pc in the garage, so I wanted to have a pc dedicated only to the router and will be using Mach 2. I really dont want to spend money if I dont have to (we all feel that way lol) what do you all recomend? Minimum and optimum requirements?

Thanks in advance