What graphics card do you now have? The reason I ask, is that if you do not make a significant jump of several tiers in gpu power, you are likely to be disappointed.
Your 2500K is as good as it gets for gaming. It will drive ANY graphics configuration well.
I agree with going to a larger monitor. You will see more of the game at one time. Currently 1080P seems to be the resolution of choice. If your budget is $1000 or so, then 2560 x 1600 is great. At 1080P, the load on the graphics card is the same, regardless of the size of the monitor, be it 22" or 27" or even a 60"TV. Something like 24" would be good for a desktop user. Plan on keeping your smaller monitor if you can fit it on your desktop. It is most useful to hold static stuff like e-mail,performance m onitors...etc. It will not hurt gaming performance on the main monitor.
As to which monitor, I like Samsung. They make the panels for many, if not most, monitor vendors. I think they keep the best samples for themselves. I have never seen a dead pixel with a Samsung monitor. You will pay about $230 for a good 24" one. Go ahead and do it. A good monitor will last you through several generations of PC's. You will see it and touch it every day, so get the best you feel comfortable paying for.
Here is one for example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16824001388
A GTX560ti will drive a 1080P well. A GTX570 is really about as good as you need to be. The 6950 and 6970 are the equivalent cards from ati. Do not worry much about minor differences and benchmarks. The graphics card business is very competitive so you get what you pay for. I would also avoid multi-gpu solutions. They may show well in FPS benchmarks but are prone to microstuttering which is very annoying.