Desktop: Series 3, 4, 5 Chipsets (X38/x48/x58) & associated CPU w. intel VT
Servers: 5000 / 5500 Series Server board & associated Xeon 54xx or 55xx Series.
I've bought 1 y ago: Intel S5000VSA mobo + 2x Xeon 5405 + 8GB DDR2 800 FBDIMM and Hyper-V is runing fine.
If you'll go for a purchase now: go for intel S55xx or S5520 Based server board, 2x Xeon 55xx (Nehalem) + 8 or more GB DDR3. That will rock!