[CentOS] Question on upgrading hardware

For centos 5.x

Two questions regarding upgrading hardware or system.

1- If I were to shut off the server and add more ram, switch out ram, remove one of the two cpus, or replace the two cpus with bigger or smaller ones.....Would I just be able to restart the server or would I have to do something with the configuration somewhere?

2- If I built another system, slightly different motherboard but most of the rest the same as far as hardware, would taking a drive from the old system and adding it to the new system work correctly and recognize what it needs?

Thanks for any advice on the upgrading.