Ah. It's kind of scary that private RSS of each process would be 25MB, but it's certainly possible. I assume you've disabled every module you don't need?

If you have some free time try deploying your LAMP stack with Buildroot and uClibc. The application size ends up being around an order of magnitude smaller, but i've never bothered checking if private RSS on clunky apps like PHP or Perl is minimized at all.

edit Also for prefork we used to have some scripts that would monitor processes to see if they 'went crazy' and wouldn't ever return, and reap those processes so Apache would fork a new one so we didn't have to restart the whole server. When you're under peak load and you restart a server and it sends all those clients to all your other servers which are already almost at their peak things get very nasty very quickly. I know you only have the one VPS here but for applications with many servers it can be handy.