I have the ASROCK H81 BTC with 4X 290X Cards running so far I ran out of riser cables so I will soon be able to test adding a 5th and 6th GPU. For Power Draw between my 2 rigs with these cards I would say they average about 300-350 Watts per GPU under full load my 2 GPU rig with i7 CPU is pulling a total of 750 Watts from the wall under full load. I would say you need about 1750 Watts for the cards plus additional overhead to cover the CPU Hard Drive and MB depending on what you are using so I would suggest you go with 1 X 1300 Watt PSU's put 3 GPU's on that and 1X 1000 Watt with 2 GPU's and System Components you should have enough power to handle everything with some room. As for brand I have tried Antec, Enermax, EVGA all work well. the Antec PSU's actually have a link cable to fire up both PSU's when the motherboard turns on eliminating the need for any PSU adapter cables which is nice but they are harder to find I have 2 X 1300 Platinum units (
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371064) there back in stock again
For CPU I have most of my rigs running on a i3
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116947since its the lowest power draw of 35W not the cheapest but work well with my setup
You could also use this much cheaper but 55W
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116950As for RAM you need at minimum 8GB to run these cards I am going to do some further testing with 16GB to see if i can get any performance boost with my multi card rig. I have one rig with 2 290x's, i7 CPU, 8GB RAM, 1300 Watt PSU running Windows 7 I am getting 960 Kh Each
My other Rig has 4 290X, i3, 8GB RAM, 2 X 1300Watt PSU's Running Windows 8.1 and I am only getting between 820-890 kh per card when i mirror the settings from my 2 GPU rig I have one card spike up to 960Kh and all other cards drop to 770KH
I still have more tinkering to do but for now i left it this was since its the most stable and highest has rate overall cards.
I hope this helps I spent many days reading forums to get this setup. I do not have the Tri X cards so there is a chance they might draw a little more power with the 3 fans.