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.