6990s are not worth it. Period.
They are noisy too -- go ahead and google it. I was tempted to buy one used for $600 from someone locally, then I found out they A) run hot and B) run loud.
Oh, and they have a lousy $/MH.
Unless there is a motherboard/PSU drought where you live, skip the 6990.
It's all noisy when your running as many fans/cards as I am. Are you saying that cause you can't afford them or are you being serious? You have a limited amount of space with all motherboards, so you have to determine what combination is going to give you the maximum MH/s, and at this point in time the 6990s that are what give you the maximum potential off a standard motherboard. If you're on a budget and just slowly trying to get into this then sure there are more affordable options. I think if you could modify a main board to have 10 wide angle pci express slots,then you could make the argument that it's better to go cheaper and duplicate. However, when you are working with a limited amount of space (most standard atx motherboards) you have to squeeze the most power you can into that space, to maximize potential, and productivity. Whats better, having 10 6950s and 2 different computers (mobo,cpu,ram,harddrive, and fans, not counting cases cause I know some people like to go ghetto.)Or having 4 6990s that do the same job with half the power, Whats more cost efficient with space,time,energy? It will look like your saving money in the beginning, but you'll end up regretting it in the end when difficulty goes up, and your sitting there wishing you went for the better investment.
10 6950 (ref models) can do 425 mh/s.. so that would be 4,250 mh/s.
4 6990s (any model) can do, lets say 800 mh/s so that is 3200 mh/s.
looks like we need to toss in two more 6990s to get over 4,250 mh/s solidly.
lets say the 6990s need 400 watts each. 6 x 400 = 2400
lets say the 6950s need 200 watts each. 10 x 200 = 2000
price of a 6990 is about 700? Sounds ok to me. 6 x 700 = 4200
price of a 6950 ref model is about 260 lets say. 10 x 260 = 2600
6990 at retail price just do not make sense at all to me. especially when ram and proc are dirt cheap.
mother boards might add some in cost but 750-1000 watt power supplies aint that expensive. HDs can
be had for dirt cheap too. so even adding that on in cost you will still have money left over.
now 6 - 6990s.. getting that all to fit and work on only two motherboards aint so tough but those power
supplies will kill you. You are not finding those under 100 bucks. etc..