Our System Processes shows like this:

top - 19:22:55 up 61 days, 7:29, 0 users, load average: 2.00, 4.26, 4.24
Tasks: 190 total, 2 running, 188 sleeping, 0 stopped, 0 zombie
Cpu(s): 23.4%us, 1.8%sy, 0.0%ni, 73.7%id, 0.3%wa, 0.1%hi, 0.7%si, 0.0%st
Mem: 2074884k total, 613180k used, 1461704k free, 8180k buffers
Swap: 12191792k total, 135296k used, 12056496k free, 31484k cached

As I haven't any clue about these things, how do these looks like to you? Everything ok?

Looks good to me, although I am not expert.

Is there any "system processes for dummies" to explain what those different figures really mean?

Like "Cpu(s): 23.4%us", what that mean in a real world?

Is this a dedicated server? How many users do you typically have online in any 15 minute period?

Two dedicated servers, one for database and another for the other things. But I have two different IPB communities on these two servers.

This forum has like 400 members online within 15 min.

It depends how many cores you have, If you got more than 2 cores load is fine.

Yes Indeed. 2.0 for server load with a dual core server means that the each core is at 100% load which is fine for everyday usage.