First enable the CentOS Extras Repo:
$ sudo yum install epel-release
Then install dkms:
$ sudo install dkms
Install required development tools:
$ sudo yum groupinstall "Development Tools"
sudo yum install kernel-devel
The use of DKMS can be recommended highly enough as it allow the VB kernel modules to be recompiled automatically as you upgrade the kernel.
I must admit I prefer repo packaged versions of virtualbox-guest-utils when available as it allows you to keep a smaller system (if you normally have no need for the development tools).
Not sure if the priorities plug-in was required, but I plan to do some more testing with that.
Repoforge appears to be the new RPMForge?