When I first experimented with HyperV I used a cheap HP Proliant ML115 running a quad core Xeon. It runs smoothly with 2 guest operating systems, 2008R2 for Terminal Services licensing and a 2003R2 running a webserver. To my surprise, the processor doesn't support virtualization but it works fine! (I use it for my licensing server). I tried doing the same thing with another ML115, this time with a Athlon 64 but Server 2008 refuses to install. XP, Server 2003 and Ubuntu works fine though.
Whatever you do, make sure you have one guest per core! (Save one for your host too!)