Now without further due let me introduce one of the charts available in the Microsoft web site which explains which edition to choose.

Apart from server consolidation some of the other areas where you can use HYPER-V are,
* Test and Development
* Server Consolidation
* Branch Office Consolidation
* Hosted Desktop Virtualization (VDI)
Microsoft free HYPER Visor is good option for testing and R&D. If you are planning to consolidate more than 4 servers in one physical server then moving to Data center version will do huge cost saving to you. More information of these licensing and how to maximize your investment on this HYPER-V can be get on Tech.Ed 2010. Look forward to see ya in there.