[CentOS-virt] Xen Database vms

Are "centralized" database (SQL) servers best left out of virtualization and committed to their own hardware like the "old days" or are there some guidelines one should consider in setting them up? I'm not talking mega-large recordsets, but large enough to handle multiple years of CRMs and intensive querying, Accounting Systems and so forth.

Fundamental website CMS database is a no-sweat issue.

This would be on plain vanilla CentOS/Xen stock. All usual SQL server flavors considered. I'm trying to get an idea of where I am ultimately heading in my LANs.