MaxClients and MaxServers

Have you considered lighttpd or nginx? They handle large numbers of concurrent users with much less config tweaking.

apache does just fine too with minimal configuration.

One thing to understand: Serving 50 requests in parallel FAST is better than trying to serve 500 requests in parallel slowly. In first case more requests will "wait in queue", but they'll wait a tiny fraction of second; in latter, they may be stuck for a second or more, causing the queue to build up and explode.

Seriously, get apache-worker with maybe 100 "minimum" threads and burstable to 250 or so, and set up number of your dynamic content handlers appropriately lower (50 or less). Most of your requests should be for static data like images, so the real "narrow throat" (and also, the main memory eater) are the php/python/whateveryouwilluse handlers.

If you don't use modphp, 150 is OK, but I wouldn't increase it any further. But if you don't use modphp, there really isn't any reason to use Apache in the first place. Both lighttpd and nginx can serve static files to 10K+ clients at a time without breaking a sweat, and any dynamic stuff (PHP, Rails, Django) would run as a separate process anyway.