about dual fermi, look at dual gt200...
http://www.xbitlabs.com/articles/vid...0_5.html#sect0
285 = 160W
295 = 200W

480 = 280W (rumored)
495 = 300W (tdp limit)

going above 300W is painful... you need very expensive cooling above that point, and you break compatibility with pci-e specs, so i think thats unlikely.

so for gt200 nvidia had to cut down a single gt200 card from 160W to 100W, a 60% reduction, and they achieved that by lowering clocks to 576/1200/1000 from 648/1476/1242 which means a reduction of 12%/9%/24% and they cut the memory hub down from 8 to 7 channels, which means a bw reduction of 13%. so id say on average they cut its specs down by around 25% and that allowed them to reduce the power consumption by 60%. thats a very good trade off!

for gf100 they will have to cut down power consumption by ~90%... we dont know how gf100 scales, how well does it clock with lower voltages, and how fast does the power consumption drop when they reduce voltages? gt200 did very well there, if we assume its the same for gf100 then we need to cut the perf specs by 90% or 1/3 more than for gt200. in other words, the specs need to go down not 25% but 33%. thats a very optimistic guess, and in this case a 490 would perform about 1.5x as fast as a 480, so 50% faster, best case! since sli doesnt always scale lineraly, the average perf boost would probably be around 40% or less.

a dual gf100 card would easily cost 1000$ if not more, would be even hotter and power hungry than a 480, and would only be 40% faster... i think this doesnt really make sense... from a business point of view nvidia will make more money by selling single gpu cards, as even if they offer us a good deal for the dual gpu card, it will be very expensive and too expensive for 99% of us for sure. the only reason to do this would be pr...

but if you followed the numbers, you will notice that, assuming the numbers we heard are true, and a 480 is only marginally faster than a 5870, a 490 would barely beat the 5970... it would, but not by much... and it would cost a lot more... and amd is prepping 850mhz+ clocked 5970 which are 10-20% faster than current 5970s, so a dual gf100 card doesnt make much sense at all right now... they have a limited supply for gf100 chips to begin with, they are rumored to have slim margins even at high prices, and they are rumored to have tdp problems... im sure there will be a dual gf100 card, but i dont think we will see this anytime soon... Q3 the earliest if you ask me...