how many FLOPS for the buck?

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i was recently talking with a friend and the subject of practical computing came up and the price of implementing any reasonably fast system for private people,

now i immediately gravitated toward the concept used by bitcoin farmers and the like, they use a large amount of cheap GPU's with CUDA or the amd alternative and it saves them a lot of the costs pr practical FLOP operation, now this requires a sense of the pricing of individual GPU's i was thinking of the HD6950 for the price of roughly 180$ you get 2.25 TFLOPs at single precision,

does anyone know of a better GPU in regards to FLOPs for the dollar?

any experiences in noise management and heat controls? (i was thinking of watercooling them but that depends on the radiator size required, been a long time since i watercooled anything)
we would use 4 cards if possible, i already have some low latency ram for it to use, ddr3 dual channel.

second hand cards I'm assuming, can't find any new 6950's for under ~240 here, they still seem to be the cheapest/flop, although you could wait for the 7970/50's, 3.8 and 2.9 Tflops respectively, to come down in price. As for cooling, unless you're planning on overclocking, stock should be fine 6k series are very quiet assuming case air flow is decent, and wc loops can get very annoying especially if a gpu dies or something, also although CUDA is awesome, you need to pay alot more to get the equivalent performance nvidia cards as they tend to throttle the cuda performance of their consumer level cards to create a market for their professional level cards.

i buy without vat and sales tax as a company, as such they should be about the 200 dollar mark, a 180 might be a tad too low,
thing is i have a 6950 in my gaming rig, wonderfull card and as you say decently quiet, you might be right in stock cooling being more than adequate but when seating 4 cards there would be some thermal buildup, most quad rigs suffer from this and rely on a huge airflow from 3rd party fans, might be possible, certainly cheaper.

AMD has their own version of CUDA(alternative not derivative)
second hand cards is an option, if i can find 4 with similar enough specs to make crossfire feasible, different brand cards is never a good idea for one.

In terms of crossfire/sli compatibility, as long as its the same physical chip in my experience brand doesn't matter, just as long as they have the same or similar clockspeeds, and CF/sli support means nothing if you're not talking about graphics processing.

also in terms of cooling, a larger form factor board that allows for space between each card could save you from going down the wc path but I'd assume that it would add extra cost etc... I've seen cases that have breakout boards with ribbon cables (for PCIe slots) that might allow for extra card spacing, can't remember the name off the top of my head but it could be something to look into.

hmm there are some people that have had troubles using different brands but if they observed the same memory and core clock i dont know.

as for the case, it will be custom built, the price is the same and since form faktor is of some importance we will have much more freedom this way,

PCI-E breakouts would certainly allow for proper spacing, it would also allow us to fit the gpus with a direct air intake instead of using case air.

so far i havent been able to find any great amount of used 6950'ies, they are popular and not a lot of people are selling, perhaps when the 7950 falls in price someone will.
i think i have to make an overview of the total cost with these plans, see how much it amounts to, will report back if interested.

sure, I'd be interested to see how it turns out, been playing with the idea of building something like this myself.

Just found this and immediately thought of this thread, expansion card risers >> http://www.ameri-rack.com/PCI-EXPRESS.htm

i found some similar ones not long ago,

currently we are still stuck at finding some decent second hand cards,

the motherboard has been measured and the backplate for the chassis cut out and drilled.
next up is cutting the acrylic shell and mounting the fans.