You can run on a single 733MHz with only 512Mb of RAM. This a simple 5 zone server setup with MySQL and EQ.
However you might notice a litle bit of network lag unless you use the task manager to set the priority of EQ down one level.
The ideal setup is where you have all your zone servers on separate machines on your LAN. This also helps when trying to debug the code.

My development machine is a duel 2GHz with 1Gb RAM and dual montiors.
-Edit- Spelling :p