Server Specs

We're going to be setting up a few new FreeRADIUS servers on virtual hardware. The server admin is asking me what I need for specs. (Virtually, they can allocate whatever I need) It's about 200-500 simultaneous authentications. (This is my prediction for the next 4 years, we're about 10 right now). Using NTLM-auth to talk to Active Directory, and performing PEAP. I'm guessing 1ghz of cpu, 512MB of RAM, and 30gig of HD. Suggestions, comments, criticisms?