I recommend that you use OpenVZ with HyperVM as your software, because it's cheap. The HyperVM licensing only costs $0.50 per VPS you create but that is only if you create more than 5. Although, if you wanted to make Windows VM's, then I would go for XEN.
Yes, OpenVZ and HyperVM is the cheapest out there and also reliable. You do need CentOS 5.2 as the OS and no other control panel on the server.
Virtuozzo is the best in the industry I think. However, if you go for the free solution then you should try virtual server 2005 from Microsoft.