When should you offload PHD to another server?

We are looking at putting in an XSAN. The Backup Metadata controller will also be the OD Master and the file sharing head. The current file server doesn't have a lot of load on it. If we set up about 30 PHDs will we need another machine just to be the front end for that? Would it need to be a new machine, or would a dual 1.4GHz G4 do the trick? The hope is that we can get backups done more efficiently, while also allowing users to roam between machines. There won't be a lot of users doing that, so the syncs should generally be fairly minimal. Thanks for any input.