That's kinda what I'm aiming for in general. My dev box died and I've replaced it with a quad core that's completely overpowered for what I was doing. Collapsing the unRAID system into it would also allow me to get rid of excess traffic on my network as I can migrate my torrent client into a VM on the same system.
When you do your testing are you planning on modifying the VM Memory size to see if there's a "sweet spot" for caching? I've got 4gb on the system now, it will hold up to 16 and I wouldn't object to performing RAM upgrades down the road if there is benefit to be gained from something larger than 1 or 2gb.
And of course the big question I run into is: I'm thinking about scrapping the whole system and drive array that my unRAID server is running on (2x 1.5 TB, a 1 TB, 2x .75TB and a .5TB) and grabbing a pack of new 2TB drives to build on. Problem is, I only have the one license for unRAID. If I make a new VM with a new disk pack, how do I get the data over given that there is only "one server"...