I run a forum with the following stats:
Threads: 827,238
Posts: 9,623,437
Members: 635,234
Average of users online: 4,000
My specs are as follows:
RAM: 12GB
Processor: AMD phenom || X6 1055T 6-core Processor
Harddrive: 2.5TB
We use Litespeed webserver and MyBB 1.6 and rarely get any problems...