Centos – How to change from prefork to worker MPM on CentOS 64bit

Since you're running PHP on that server, I would be cautious about using the Worker MPM. There are some known issues with Worker and PHP. The PHP Installation FAQ recommends against it.

Not more than a gig or two -- you want a little bit of swap available, just in case and because it helps things work a bit better, but if you ever get to the point where you're heavily swapping, the machine is going to be useless -- and in a real catastrophic situation, you're actually better off with a smaller swap space, because then you'll trigger the OOM killer sooner rather than later.