First off, your "inexpensive server" and "make a little money by renting it out" aims are incompatible. If you use inexpensive parts you will end up with an ultimately unreliable server, and charging people to use this will just generate badwill and you will be abused by anyone who's relying on your services.
Wow, you'd almost think I've worked for a company that tried to build inexpensive servers! :roll:
What you need to run a server; Stability. Stability. Stability.
Followed by RAM and hard drive space
CPU is less important than either of the above. You won't need a decent graphics card - onboard is fine. I'd strongly suggest either using nLite to cut down a copy of Windows to remove stuff you don't need to reduce overhead, or use Linux.