Server Sizing Ubuntu 8.04 LTS Zimbra 6

[sieggie]

i dont know if i'm on the right area, anyway my question is regarding server sizing and how to go about setting up a high availability server...i'm running a test server right now using ubuntu 8.04 32-bit with zimbra 6...we're planning on using this solution in the next coming months...the server should be good for 400 users initially and can go up to 800 to 1,000. i just want to get some ideas on what hardwares do we need to purchase (ram, cpu, harddrive, etc.)...thanks!