the i5 3570k is a cpu that can go well with any gpu. so really, the best one would be whatever is strongest, since the gpu is more imporant than the cpu. though in my opinion, your budget should have been more gpu oriented rather than cpu oriented.
If you are looking to game, you should have spent more money on the gpu like dude said. However, you didn't specify how much you cared about gaming anyway so who knows. Anyway the 6870 will be the strongest, but I have heard really great things about the 460 all around. I would suggest saving another hundred dollars, and getting a 7000 series gpu. Or a 660 if they ever come out lol
you may want to look into whether or not lucid software can be used on those gpu's (it combines the igpu of the ivybridge processor with that of the discrete gpu) you have to have a tb capable mobo but thats going to become more and more widespread. I think its pretty neat and your getting to take full advantage of the cpu and gpu!
Okay guys thanks for replies, after a bit more research I actually ordered 6850. Apparently when overclocked it is getting quite close to performance of 6870 and it is actually slightly better than GTX 460. $30 is not a big difference but I prefer not to exceed my planned budget.
Also ryfry I will check that software, I am going to buy i5 3570K and ASRock Z77 Pro 3 MoBo. Do you think it is good enough for Lucid's software?
I'd definitely get the 6870 if you could find it cheaper. I've seen it at $130. It provides a big performance jump (like 20%) over the 6850: http://www.anandtech.com/bench/GPU11/188
And the overclocking argument only applies when you're comparing an exceptionally great overclocking card (like GTX 560Ti) to an average or bad overclocking card (like GTX 570, which is average) since both cards can be OC'd. At the price you paid, the 6850 was the best choice.
Any Z77 system can use Lucid's software, btw.
It's good you didn't get the GTX 460 as that's not the real 256-bit version that you'll see in all the benchmarks.