5850, 50% lower cost, 10% lower performance in worst case scenario(clock both up as far as possible most games are 5-8% apart due to a few less shaders, a couple or 10-12% apart).
A 5850, if you get one with voltage control which is pretty key to getting the highest overclocks, you're talking about a 30% overclock pretty much minimum, 40-45% max, and at that speed it will be faster than a 4870x2 without any of the dual gpu issues that crop up here and there. A overvolted max overclock 5850 will use 20W idle, and around 200W load(150W stock, its as you bump voltage power usage goes up a lot). A 4870x2 uses closer to 100W idle and 280-300W load IIRC, once overclocked it will use a good deal more than that aswell.
I went from a 4870x2 to a 5850, its not massively faster, its much easier, my room is FAR cooler in summer on top of all that its a lot quieter aswell.
You honestly won't notice the difference between a 5850/5870, the price increase for minimal performance gain is just not close to worth it.
There literally isn't another option other than those two cards for same/more performance and a decrease in power. A 470gtx will again rival it for performance, should easily beat it overclocked, but stock power use is around 215-230W depending on the version/voltage used, and heavily overclocked and volted will be up above 300W.
The 6870/6850/6770 are due end of next month though so now's a fairly bad time to buy, good deals on the last few 5850/5870/5970/5770 will be available at some point soon, and a 6850 might offer another 30% speed for around the same power draw which would likely give you a card you will see is noticably faster than the 4870x2.
I don't think they can bump up the 6770 performance enough to really get that close to a 4870x2, but you never know, improved 6770, massive overclock it could be very close and use a heck of a lot less power(closer to 100W than 200W).
Its worth waiting for sure. Launch is 12th Oct, availability from 25th onward is the fairly trustworthy rumour at the moment.