After reading through the Citrix Xen and RHEL manuals, I've decided to go with the KVM virtual hosts that comes with CentOS. Can't see any advantages of going with Citrix since the RHEL manuals are pretty clear on the steps need to setup a virtual host.

The question now is I'm planning to run this on a AMD Phenom II X3 to begin with due to cost reason. Since there's a restriction on doing Live Migration between hosts on different CPUs, would there be restrictions also for transferring the whole system onto a new server running an Intel Xeon later on?

Also not too clear in the document for KVM is what would happen to the original host OS after installing a VM guest? Can I run things on it still? Should I setup the Postfix server first, then setup the VM? Or even something basic like the Postfix has to be running in VM?

Generally speaking you want to cut down your vmhost as far as possible and have as little unnecessary items running as possible. Don't look at guests as "running a program" look at guests as "running another server". The host and guests are all their own servers, but anything that compromises the host can automatically compromise the guests, so there should be ~nothing~ running on the host that would leave additional potential for a compromise. A mail server is a definitely something you would not want running on the host. You don't need to give a low volume mail server 16gb of ram and 4ghz of processor. Check out some of the offerings at linode or other vm providers... 128mb of ram, 1ghz, 10gb hd. Provision machines for what they need, not like a physical server.

How about if I use an AMD processor now, would I be able to transfer my VMs to a box running Intel processor later on?

of course. only live migrations don't work (actually they do work, but with some restrictions)