Hello Froggio
I don't know why you have the Centos kernels, however even if you had the sme-server variants you would still have trouble at inital boot after a YUM upgrade.
We also run C3's and the later Nehemiahs - they won't run the SMP kernel. Unfortunately after a YUM update, the smp kernel is always set as the default in grub.conf.
Simply boot up to the penguin splash screen and press the space bar. Then choose the non smp kernel - (we currently use 2.6.9-42.0.10.EL) and your box should come up OK.
Best
Selintra