I am new and just installed ipb board. In a year or so i would like to see my forum grow and hopefully have thousands of members online at one time...how do I know if my server will handle this?

I use ix webhosting...if I am having problems I wonder if I can just paythem more to upgrade me?

Any one want to help a newbie out?

Usually your host will contact you if you exceed your resources on a shared host.
From there, you usually go to a VPS, then a Dedicated server.

what is vps?

Virtual Private Server

What each server can handle depends on a lot of things (how many other sites are on the server and how many people online on each site, how much memory and what kind of CPU's are installed, if database and webserver run from the same box, and so on). As a general rule of thumb I'd say

~100 on a Shared server
~200 on a VPS
~400 on a single dedicated
~800-1000 on split dedicated (web/database separate boxes)

These are just numbers to consider - your host will tell you if you exceed their resource capabilities and usually offer to upgrade you to a new plan.