cPanel alone would take a lot of resources....especially RAM.
I'd say the lowest you can do, is a Pentium 3 / celeron with 512mb of ram, and a decent uplink (10/10mb or better).
Linux without a GUI is a must.
And if course, you wouldn't be able to host many sites. Maybe a dozen or two, depending on just how much the sites are used. Obviously, if 2-3 people use them, then that it's going to put much strain on the resources.
I'd say it depends on the number of "purely active" accounts.