[CentOS] does anyone have experience with clusters?

Hi all,

I want to start experimenting with clusters, and I would like to use
normal desktop grade hardware for this. I have some extra PC
components lying around, enough to build 3 - 4 moderate desktops with
a PIV / C2D CPU & 512MB - 1GB RAM each. All the machines should have
at least a 100MB NIC, but I can add a gigabit NIC to the machines that
doesn't have it if need be.

I have used Linux Heartbeat to failover a MySQL cluster before, and
it's actually been running very well for about 3 years now. But, I
want to start looking @ total clustering, with DRBD - where everything
(not just MySQL) is being replicated across the 4 machines. I'm also
intereated in load balancing.

Has anyone done anything like this?

The cluster will be running / hosting a few XEN Virtual Machines, and
idealy if one PC / server dies (hardware failure / power failure /
etc), then on of the other machines should still be running the
virtual machines, i.e. no downtime. Is this possible? Can somone steer
me into a direction where I can get more info on this?

My aim: to use common, cheap PC equipment & CentOS for the project, I
don't want to spend any more money, and want to use what I have.

-- 

Kind Regards
Rudi Ahlers