It has to do with how efficient these CPUs are at completing their tasks. The i7 970 uses an older architecture that cannot get as much work done per clock cycle compared to the newer i7 2600k, even with the 2 extra cores. The i7 970 is priced higher due to being on a more enthusiast oriented platform, LGA 1366, where a much higher price premium is expected, and it is one of the few hexacore CPUs available from intel. The other two hexacores are Extreme Edition CPUs priced around $1000. The i7 970 was a very powerful CPU when it came out around 3 years ago, and its price really hasn't dropped that much, even after better CPUs have come out.
As far as value goes, there is no reason to get the i7 970, the 2600k is faster in every way, and comes on a cheaper, and more modern platform, LGA 1155. LGA 1155 has a further upgrade path to Ivy Bridge, LGA 1366 is a dead socket, it's getting replaced by LGA 2011 in 2012. The only advantage 1366 has over 1155 right now is higher memory bandwidth with the triple channel memory configuration, and very few applications will take advantage of that. 1366 also has more PCI Express lanes than 1155, but that is only really relevant if you have a really high end triple GPU or quad GPU setup.
The 5850 is compatible with any LGA 1155 board. For a 2600k you would want a board with overclocking capabilities, so go for one based on the P67 or Z68 chipset. The Generation 3 Z68 boards will have PCI Express 3.0 compatibility with the future Ivy Bridge CPUs, but no graphics card in the foreseeable future is going to need that much bandwidth, high end cards are only starting to saturate PCI Express 1.0 bandwidth right now, with only the dual GPU cards coming anywhere close to saturating PCI Express 2.0's bandwidth.
Not really, the Z68's only extra features over P67are Quick Sync, which is only relevant if you are using intel's integrated graphics, and SSD caching. Z68 is a little more futureproof with the PCI-E 3.0 compatibility on some boards. If you go for a P67 board you may want to avoid the ASUS ones, as they have been known to be a bit flaky, their Z68s are okay though. Most boards from ASUS, Gigabyte, and MSI are quite solid.
also keep in mind that the memory controllers are built into the CPUs, so the 970 has a triple channel controller vs the 2600's dual channel controller. Features like these add to the costs of the 970 even though the 2600 can execute instructions faster. If the memory bandwidth is really important you have to pay more for it.