Right now very few games can take advantage of more than 3 threads so you should do well with the Intel® Core™ I5 2500k. Of course look at the software that you are going to be running if it will support more than 3 threads you may want to look at another processor.
While this would be the ideal setup out of the listed parts, I'm guessing it's out of budget. By like $100-200. Also, "good for 1680x1050"? Really? The 6850 is more than powerful enough to play the vast majority of games at 1920x1080 with high/max details.
For a gaming machine, the GPU is much more important than the CPU. While the i5-2500K is a great processor, it's also pricey. If affording it would require a huge sacrifice in terms of GPU power (dropping from the 6850 to 5670), then it's not a good idea. The 5670 would only be good for playing the most basic games, not the majority of games out there. The X4 955 (the cheaper, just as powerful brother to the 965) would be just fine.
I would also check out the ASRock 870 Extreme3 for the board. It's cheaper than most Asus boards, but it's high quaility, high performance and packed with features.
1280x1024 is pretty low. I'd actually guess that the 5670 would play them at that resolution very well. Tom's monthly GPU review states that the 5670 gives "excellent" performance at 1680x1050. If you drop the resolution, I'm sure you'd get playable FPS with max details. However, it's still not a good idea.
You could possibly afford the i5-2500K and still get a GPU that will max out those games. I'd look for the ASRock P67 Extreme4 as the board. It's typically $40 or so cheaper than Asus boards. That would allow you to get the HD 5770 or possibly even a GTX 460 768 MB.