I'd recommend against running multiple servers unless it's absolutely necessary (and it only is when you have 1000+ simultaneous players, if even then). We run a single box with the following specs:
- Dual 2.4GHz Xeon
- 2GB of RAM
- 100mb/s pipe
Like I think I've said before, at 150 simultaneous players we're not even hitting 20% CPU usage, so this box is more than what you need for anything EQEmu-related.