Budget isn't an issue (within reason), I'm looking to set up a virtualised gaming rig much like yourself. There'll be a Windows 7 (64-bit ent) VM that'll need as much 3D gaming grunt as possible and a couple of other VMs that need some acceleration to be responsive and usable (possibly 3D too). Naturally I want to go for as much power as possible, I plan to use the machine for current and next gen gaming.
Depends on your intended resolution. If all you want is 1080 resolution capability, GTX480/Q6000 will deliver. Granted, my eyes don't seem to see things the way most people's do (more pixels, not as many frames per second, or so it seems), but I happily completed Crysis+Warhead on a T221 3840x2400 in a VM on my Quadrified GTX480. On the same physical host, my wife was finding Borderlands 2 unplayably bad at 2560x1600 (so I temporarily put a HD7970 in her VM (and yes, it crashed the host when you try to reboot the VM - I'm hoping the ATI pollution is going to be temporary) and kept the 480 for my VM).
If you need more than 2 VMs with 3D acceleration on that motherboard, you are going to have to use something like a GTX690, given it only has 2 PCIe x16 slots.
I thought about converting a GTX 480 into a Q6000 as they're fairly cheap to pick up used on ebay, however I'm not sure which brand would follow reference design to make the modification less of a headache.
They all do / it doesn't matter.
What wasn't made clear earlier was that a Q6000 clone (GTX 470 / 480) can be used as vSGA with 6 guests, that's a great piece of information for those looking to accelerate 3D on multiple VMs on a budget
Quick googling some of the terms in the article from memoring yields this:
http://www.simonlong.co.uk/blog/2012/10/25/vmware-view-3d-gaming-experience/Note that this is not exactly high-performance gaming - it may be 6 clients, but they are 800x600@25fps.
I should also point out that I don't use ESXi for this stuff, so you might want to start on the cheap with a modified 470 or 480 (performance difference between them is pretty negligible, and a 470 will likely outperform a Q6000 as long as you don't run out of VRAM) as a proof of concept before you commit to a more expensive piece of kit like a 680/770 or a 690. I should perhaps also point out (hint:nudge) that there is currently a quadrified GTX470 on eBay at the moment.
What would also be handy to know, and again I assume this is probably beyond scope of this thread, would be if multiple Q6000 clones can be added to a system, one passthrough'd to one VM directly for as much acceleration as possible, and a second Q6000 clone distributing as vSGA between remaining VMs? GTX 480s can be picked up second hand for around £100 each on eBay so they'd make a great price vs benefit starting point.
I see no reason why you couldn't use one card for vSGA and one for vDGA. Just bear in mind that you won't be getting video directly out of your vSGA cards - those VMs will feed you a compressed video stream of the desktop that you will have to decode on another machine. The problem with this being that you need another machine as a terminal (unless you use your vDGA machine as a terminal for it, which would work I suppose, but it gets a bit recursive).
But as I said before - I am not an ESXi user, and while I would expect their solution to be a little more polished than Xen, I suspect you will also have a lot less community support to fall back on if it doesn't work out of the box. Also, last time I checked vDGA was treated as an experimental feature.
The GTX 680 I was secretly hoping you would have found a solution by now, but as with all things of this nature, it wouldn't be too easy or everyone would be doing this to their cards
It could be that I just have a weird GTX680, or there is some OS/environmental issue that is manifesting as the problem I mentioned. One of the guys on the Xen list modified a completely standard GTX680, and his works fine in all modes, so my DVI issues are most likely just a bizzare quirk of my system configuration.