I'm a little hesitant if I want to go with the high-end CPU. Should I lower my CPU and up my GPU? I considered the i7-2600k because CPUs generally do not need to be upgraded as often as GPUs, and each CPU tends to last 2-3 GPU upgrades. However, I'm definitely not an expert in this subject, so I am open to any and all suggestions.
Go with the i5 2500k for gaming, the 2600k isn't necessary for gaming unless you need the hyperthreading for something else. (not to mention the 2500k saves you 100 bucks that you can put towards your GPU)
you will probably need to replace your RAM as well if you have old DDR2 memory. also make sure your system is crossfire-ready with sufficient power.
as future games utilize more threads, the hyperthreading on the i7 will come in handy. but for current games, the i5 provides near identical performance. I would suggest you stick with the i5, it provides great performance itself, and you can always upgrade it later to ivy bridge or whatever.
regarding GPU, I would suggest a single 6950 if you are using a single 1080p monitor. great performance at a great price, and no dealing with crossfire hassles.
Whats your resolution? Your graphics solution will depend on that as much as your budget. Also, not to sound condescending, but make sure you can fit what you buy into your case and support it with your PSU, both wattage and cables.
The thing about the GPU, the CF 6870 is giving me a lot more performances than a single 6950. A CF 6850 is similar to the 6870 with a few minor FPS in some benchmarks, but the $40 difference is just leading me to the higher model.
I don't have much experience with AMD processors, but from what I see, you should be fine on that. Unless your GTS 250 is being bottlenecked by your processor (which I highly doubt is the case), you will see absolutely no performance increase from a processor upgrade.
Your CPU was released in 2008. It's getting old, but the statistics still look good. I would advise a GPU upgrade. However, I would advise dual http://www.newegg.com/Product/Product.aspx?Item=N82E168... as opposed to what you previously mentioned. The GTX 460 outperforms the HD 6850, and the one I linked is an OC version, meaning you'll see even better performance.
Just make sure that, no matter what you do, your PSU can handle the load. An upgrade does no good if you can't feed it enough power. For either of these cards, I'd recommend nothing less than a 700w PSU if you're going with dual GPUs.