I'm in the process of building a pc for live mixing, and have been trying different hardware combinations. My experience so far: vMix performs very well in many different settings!

My goal was/is to get many camery inputs as cheap as possible. So i grabbed a number of different capture cards and put them all into the same machine: one intensity pro, two AVerTV capture HD (H727), two AVermedia game broadcaster HD (C127) - ok, i know, there are quadruple input cards as well... but they are more expensive.
The first problem was to find a mainboard that has enough pci-e x1 slots for the purpose - i went with an Asrock H61DEL (cheap one, got it for € 45), which has 5x pci-e x1 and 1x pci-e x16. I hoped, that i wouldn't need a dedicated graphics board (thus leaving the sixth pci-e slot for yet another capture card... *g*), because i selected intels i5 3470 with integrated HD graphics engine 2500. And i think i was right! I've been testing the system with 4 gb of ram on win7 x64 with as much as 5+2 inputs (5 hd, and 2 sd - the h727 boards actually have two independent input circuits!) and vMix 9.0.120 still performed rock solid, cpu ranging at 20%.

There are some caveats though:
My intensity pro board is rather picky about hdmi inputs - it wouldn't connect to my Sony SR1, although all other cards didn't have any problems there.
The H727 boards don't support 1080@50p, at least officially. I have no 50p sources, so i couldn't test. With 50i there are no problems, not even with the Sony SR1 that was rejeced by the intensity boards.
The H727 have two input ports, one is HDMI, the other component/composite/s-video, and there is also a tv-tuner that does analoge and dvb-t - these are the physical inputs. It has two capture circuits, one for SD and one for HD capture, that show in vMix as "primary" and "secondary" inputs, where HD is secondary. You can use both at the same time, but they can not be both set to the same physical input (eg. both grabbing HDMI input).
The C127 boards just do their jobs, even in 50p.

I'll do more testing at the weekend, and then i'll probably also have some photos...

The next step will be integration with grandvj on the same machine - i'll probably need to add a graphics card then, to get a third screen attached (yes, i know that vMix GUI has to be on the same gpu as its output...).