I have been doing some light research on these two cards, the 5850 is slightly cheaper than that 6870, but from what I could tell the 6870 is supposed to use less power and be slightly faster. However, some reviews I have read said that the card (perhaps its the brand maker) will get really hot. I know it says I currently have a 5850 in my system specs, however, this is not that case anymore as I got unlucky with my card. I would go Nvidia, but the 470 uses too much power, but I would consider a 460 gtx.
With all the cards you mentioned, only the GTX 470 is really out there in terms of power consumption, but it is the highest performing of the bunch. Between the 6870 and 5850, I would get the 6870 for it's improved DirectX 11 tesselation performance and outstanding power consumption numbers. Don't expect it to overclock very much, and really look hard for one with a good cooler that doesn't get loud when gaming, several do get fairly loud.
Between the GTX 460 and 6870, that would be a different discussion, but the 6870 generally outperforms the 460, unless you overclock the 460, then the differences are minimal. For the 460, there are some great options in terms of aftermarket coolers and designs. I would look at the Gigabyte Superoverclock, the ECS Black, or the MSI Hawk Talon Attack models. The 460's will have even better DirectX 11 tesselation capabilities, better texture filtering, and PhysX.
Notice that on average, the 470 had higher average FPS, while the 6870 had higher min FPS. Given it's the low FPS times that cause the issues, you could consider that a win for the 6870. It's sometimes hard to tell without first hand experience.
from what I can see, the 6870 doesn't OC that well at the moment, unlike the 6850 or 5850. Also I heard the new drivers for the 6800 series are not the best yet.
The EVGA FTW is the highest factory overclocked GTX460. They build it with top of the line components to ensure that it can withstand the stress of higher clock speeds.
The 6870 was reference clocked near the top of it's potential, so no, it doesn't overclock very well. It also doesn't scale well when overclocking, compared to the GTX460.
Another issue with the 6870 is the fact that they use an image quality "optimization" that increased it's benchmark performance numbers by 10% at the expense of image quality. Keep that in mind when comparing benchmark scores.
Tom's Hardware had a nice write up on the issue: http://www.tomshardware.com/reviews/geforce-gtx-570-gf1...
"For the time being, we're going to have to leave everything at default and point out the observed and confirmed image quality issue currently affecting Radeon HD 6800-series cards. This may or may not become a factor in your buying decision, but right now, the bottom line is that Nvidia offers better texture filtering, regardless of whether you’re one of the folks who can appreciate it."