I've recently moved from a VPS to a dedicated server and need some advice about my current setup.
Immediately I ran into problems with memory as what I didn't factor was that the VPS was allowing memory bursting. Adding more RAM immediately solved the problem.

Running Apache/1.3.27 (Unix) (Red-Hat/Linux) mod_perl/1.26 PHP/4.3.3 FrontPage/5.0.2 mod_ssl/2.8.12 OpenSSL/0.9.6b
Celeron 2.4 Ghz (400MHz FSB)

Current snapshot of top:
90 processes: 84 sleeping, 1 running, 0 zombie, 5 stopped
CPU states: 1.5% user, 2.5% system, 0.0% nice, 95.8% idle
Mem: 524288K av, 305452K used, 218836K free, 23572K shrd, 0K buff
Swap: 524288K av, 0K used, 524288K free 104688K cached

The sites I host are all running a php/mysql cart software.

1) the average apache processess are 17M.
Is this size about right?

2) how do I determine the resource utilisation by any one particular site I host?
Importantly I want to know how much memory a particular site is using at any given point. The server runs Ensim control panel and it gives me stats on bandwidth but not on mem usage.

3) is there a way to determine the number of current clients?
i want to know this so I can set MaxClients accurately.
From my current calculations it should be setting MaxClients to around 25.
(memory = 500M, av. httpd process 17M, room for other processes)
25*17=425M + 75M for other processes

Any advice would be appreciated.