You and I are in the same boat. I need something more than what I can do with ESXi today and the hardware limitations are killing me. But for my work, I would prefer ESX over Hyper-V,but I have to go with something that is also convenient to my needs too.
The best part about hyper-v is that if your hardware has win2k8 drivers you are good to go. ideally you get something that is on the HCL, but if windows can see it, you're golden. So a world of RAID cards open up, other non-traditional NIC's, as well as video. Although, many servers builds I do I never install video drivers and just lock in 1024x768 to a KVM device.
I would recommend a dual port NIC, even if it's onboard. Either for redundancy and/or throughput needs.
Buy as many cores as you can afford, quad core or go home! Between an intel or amd, there is little difference and if we're not talking Opteron or Xeon, it's a moot point.
With that said, I'm a huge fan of SuperMicro gear, but that is I believe 100% Intel. Gigabyte has some great AM2+ and AMD3 boards, but I think that AM2/AM2+ is a little more mature IMO. Stick with AMD chipsets as they follow the standard and are typically more reliable than nVidia which is more concerned about cutting edge features. I'm not familiar with Asrock to comment.
Which leaves you with a GIGABYTE GA-MA790FXT-UD5P