What does the output of the 'free' command mean?
 

Mem: 902768 877612 25156 0 46692 646384
-/+ buffers/cache: 184536 718232
Swap: 977248 13460 963788
Total: 1880016 891072 988944

What does this mean? Does it mean I only have 902mbs of RAM? I have two 512 sticks, shouldn't this be more?

It's more like 860 MB. You can use free -m to get your count in megabytes. I'm betting you don't have a kernel with high memory support running. Kernels compiled without this option only use about 860 MB of RAM (this makes memory management much more efficient for various reasons which will be of little interest to you unless you develop operating systems).

It is probably lower than the true amount as some is being shared with your graphics card. If you don't hi spec graphics - ie if this is some kind of CLI ony server then just reduce the mount of shared memory in your BIOS.

Yes this is just a web server that hosts a vB forum. I doubt it even has a graphics card.

So nothing is odd with those numbers? I have two 512mbs sticks of RAM.