Maximum Users Supported?
 

I have a dedicated server that I run my forum on. Its the only site I have on the server.

Server Specs are:
Intel C2Q Q6600
4096 MB DDR2
RAM 250 GB
7.200RPM
CentOS 5.2
Lighttpd
XCache
MYSQL 5.0
vBulletin 3.8

What is the maximum number of users I should be able to support on my site in a 1 minute interval? My forum is heavily modified and has around 50-100 users sending requests per minute, and around 1.4k in a 30 minute interval. The site is lagging horribly and I need to determine if I need a new server.