One dedicated server for now--7200 members, 1.5million+ posts. We've hit as many as 970 users online and haven't had any performance issues, although a majority of the guest users were Yahoo Slurp at the time. Normal activity is now about 650 members/non-members online during peak hours, still no major issues.
I did get our server configured with 3.25G of memory, which has helped. Well, that and George (aka "eva2000") having helped configure my.cnf to keep it running efficiently.
I have four tables as InnoDB, which really cleared up the table locking problems we've had. We're running a Celeron 2.8GHz right now, which isn't ideal, but it IS working well for us, and within our budget (fueled 100% by donations).
No members have liked this post.