Here are my thoughts, copy and pasted because I'm lazy:
I'd choose the dual GTX 480s easily. They will far outperform anything up there now. a GTX 480 is about 20% faster than a 5870 thanks to the 197.41 drivers, and may be even greater if nVidia's numbers are correct for the 256 drivers. You also get another good 15-20% more performance in the presence of a good bit of tessellation, the major selling point of DX11.
The GTX 480 also overclocks just as well as a 5870, 150 MHz on the core being very easy with everything past that pretty difficult and extreme, but then GTX 480 gains more performance per clock when overclocked. This means that the GTX 480 has an even greater lead when overclocked than the 20% now and the 15-20% with tessellation on top of the performance now!
So in reality you get a lot of performance for the extra $100 over a 5870, but the good news doesn't stop there. SLI scales a good bit better than crossfire, with 2 cards where as crossfire scales better with 3 cards. This means that you get a 15-20% increase in scaling, crossfire scales about 70% and SLI scales about 80-90%.
So lets review:
-the GTX 480 has a 20% performance increase in current titles
-The GTX 480 has another 15-20% performance lead on top of the 20% it has now when there is heavy tessellation.
-The GTX 480 overclocks just as well as a 5870, but gains more performance from the overclock, increasing the above leads even further.
-Dual card SLI scales a good bit better than crossfire, adding to the above performance leads too.
Now I know this sounds pretty raving, but there are some draw-backs.
Heat: The GTX 480 produces a bit more heat than the 5870, though nothing too out of the ordinary unlike the common misconception out now (In all honesty the 5xxx series just runs remarkably cool). I'm not saying these things aren't hot, they are, but there is really no need to worry about it unless you have a very poorly ventilated case. The heat can be tamed very well when you increase the fan speeds too which allows for some very nice overclocking, but this leads me into my second point.
Noise: This is the most misunderstood attribute about the new GTX 4xx series. They ARE almost silent at fans speeds of ~75% and under, just like the 5870. However, when you dial up the fan speeds even more they become quite loud, I'm talking 80%+. Many people think these are insanely loud because on paper, they are, but in real life the human ear does not perceive them as loud as you would think. High frequencies sound louder too us than lower frequencies, even if the actual volume and DB/DBA levels are the same. This is why a 120mm fan at 30 DBA is silent while an 80mm fan at 30 DB is annoying. The same works here, the Delta fans on the GTX 480 produce a lower hum than the 5870 which helps balance out the noise issue. Here is what one guy said recently about his GTX 480, notice in the thread that his main concern was noise:
"Thanks a lot everyone for your help... Now after 7 years I'm back to Nvidia, so far loving the GTX 480 and honestly...people really do over do the noise complaint, my 4870 was WAY louder than this whenever I booted a game, this is standable." http://www.tomshardware.com/forum/forum2.php?config=tom...
Power Consumption: This is another overblown draw-back. The GTX 480 uses between 70-100w more power on extreme load than the 5870. That is a good bit, but wont cost you more than $40 a year on your electric bill, assuming you pay a horrendous cents/kw/h otherwise the amount is even less, since these cards are at idle for most of their lives, where the difference is minimal. This does mean that you need a bigger PSU. Dual GTX 480s require about a good 850w PSU and a 900-1000w if you plan to really crank the clocks. (Note that the PSU calculator tells me I need a 850w PSU for my i5 750 @ 4.0 GHz 1.4v, dual GTX 480s, and other misc. stuff at peak loads, but when I use their recommended load settings of 90%, much more reasonable, I get about a 775-800w recommendation) Here is a link: http://extreme.outervision.com/PSUEngine
All in all the power consumption is a pretty dumb point, since anyone who is going to spend $1000 on their GTX 480s aren't going cheap on the PSU, note a good 1000w PSU costs you ~$200 nowadays, and they won't mind the slight increase on the electric bill.
As for the heat and noise, unless you are a silent PC freak then they are also small points since you will have a well cooled case and you will be tolerant of noise levels typical of the GTX 2xx and HD 4xxx days. And if you are a silent PC freak then you are already used to sacrificing performance for your obsession.
I would recommend neither. Both the HD5850 and GTX 470 overclock to a very large degree and are much better buys. The performance difference once all the cards are fully overclocked isn't nearly worth the difference in price between the HD5850/GTX 470 vs HD5870/GTX 480. Only consider the more expensive cards if you have a lot of money you don't mind spending and/or you aren't going to OC.