The Core i5-2500K can give you higher FPS than the Phenom II X4 955 without question. How much depends on how dependent is the game on the CPU. The more dependent the game is on the CPU, the more of an increase the Core i5-2500k will give you.
Most 1st person shooter games are not dependent on the CPU; the critical word is "most". Therefore, as long as the CPU is fast enough it should not hold back your video card from giving you good performance.
Assuming the Phenom II X4 955 and the GTX 560 Ti gives you 50FPS in a game, while a Core i5-2500k gives you 60FPS in the same game with the same video card. Can you tell the difference? Do you even care about that difference?
If 50FPS is good enough, then go with the Phenom II X4 955 since it is cheaper and the performance is good enough for you. And it is easier on your wallet now. However, based on the Core i5-2500k's performance it should indicate to you that in the longer term you may be able to postpone an upgrade with the Core i5 than if you bought the Phenom II. So in the long term you can save money.