Question: We have some boxes at a reputable (expensive) hosting company.

They don't offer a simple/flexible VM solution.

All I want is the ability to create CentOS VMs (mainly) to host our (mostly) static sites (corporate etc), maintaining the big boxes for more traffic/DB intensive services.

Any suggestions.

Answer: 
Check out Slicehost
I've hosted virtual machines with <http://www.melbourne.co.uk/> and have really liked their level of service so far.