If AMD's 6- and 8-core BD chips are able to compete with the 2600k, you can kiss good-bye to $170-200 AMD chip pricing for those high end parts. AMD isn't just going to give away equal or better performance at lower prices. Therefore, even if the 8-core part can surpass the 2600k (for example), it will likely be priced higher as well (especially if the performance advantage is substantial).
On average the 2600k is about
30% faster than an X6 @ 3.3ghz. This means that even if BD improves its performance per clock by 20% over Phenom II, a 6-core BD might just barely match the 2600k. Of course Intel can easily drop the price and release its own 6- and 8-core chips.
However, with SSDs and videocards still providing the most performance increase per dollar, the importance of CPU speed will remain secondary to those 2 components next 2-3 years imho (at least not until games become a
lot more intensive). I can definitely see a Core i7 920 / 930 / 860 / 870 system @ 3.8-4.0 ghz lasting another 2-3 years for most people. Of course there is always the upgrade bug for fun .