The best, reasonable current processor is the 2500K. The 2600K is slightly better, but not worth the extra cash.
The best budget processor is the AMD x4 955. Easily overclockable.
Most new games can't use more than 3 or 4 cores, so the 1090 would be a waste if you're not using something that can take advantage of it.
The i7-900s are now outdated, and cost way more than they're worth.
On your budget, you'll probably go 2500K.
For gaming the performance crown goes to the Intel Core i5-2500K. Since very few games can take advantage of 3 or more threads the value of hyper-threading is very minimal. The Intel Core i7-2600K has hyper-threading and 100MHz and the Intel Core i5-2500K is $100 less. So asking yourself this question is the hair more performance worth the $100?