Solitaire

your going to need at least a 5960X, maybe dual Xeons.
I'm sure you know the requirements of Hyper-V far better than I do and can decide appropriately if the X99 investment is worth it on that level. However I will say do not expect gaming performance to increase at all, infact it may get
slightly worse.
As for the memory, CPU and motherboard choices, if your doing it cheaply just to get the extra threads I suppose they are ok. The main difference between the CPU's is the PCIe lanes and overclocking potential (i'm only basing that on
TTL's review). If you only ever intend to use two cards and nothing else PCIe based then 16x lanes will be fine. From my perspective 40x PCIe lanes has always been the big positive for X79/X99 and I'd be a little disappointed to miss out on that, but your needs aren't the same. I wouldn't worry about Dominator Platinums, if you aren't interested in how they look or nuts overclocking then it isn't worth the added cost. Perhaps look at the faster 2800/3000 HyperX kits though as they aren't much more expensive. Motherboards I don't know much about, all the white Asus' look the same to me but presumably they have some feature and layout differences.
XMP and memory speeds do not affect the CPU speeds, however changes to the CPU strap will affect the memory and higher rated memory may be useful in those overclocking scenario's. Generally I think most would overclock the CPU multiplier, leave the strap alone and run the memory at it's XMP settings.
JR