My Apache config for a VPS with 2Gb Ram and the following:

* CentOS

* cPanel

* nginx as a reverse proxy for static files

* APC opcode caching for PHP

min spare servers 2

max spare servers 5

- should be roughly double min spare servers

start servers 2

- should be same as min spare servers

server limit 50

- 20Mb-30Mb per process so 50x30Mb=1500Mb or 75% of ram; go higher if you dare

max clients 50

- same as server limit

max requests per child 500

- ~10x the max clients. Ideally should be much higher (up to 5000) but experiment - higher means more chance of memory leaks - (depends on your linux flavour if Apache leaks memory).