1) If this is an 11gR2 RAC installation, then 2G is *very* small. I always use 4G per VM, as mentioned in my article. If it is 11gR1, then 2G is fine.
2) Oracle Linux has nothing what so ever to do with the virtualization you use. You should use Oracle Linux for your VMs because it is supported and free and you will not get some of the problems people report with CentOS. There are some bits of the Oracle installation that need workarounds for CentOS, but work fine on RHEL and Oracle Linux. It doesn't matter which virtualization product you use, you can still use Oracle Linux as the guest OS.
3) When you talk about Oracle VM, you should remember that this is a different product to Oracle VM VirtualBox, or just VirtualBox for short. VirtualBox is even easier to use than the VMware products in my opinion. You can see how I use it here:http://www.oracle-base.com/articles/11g ... ualbox.php