If you think you need to upgrade then upgrade. You will need to change the mobo and RAM to upgrade to Intel's Ivy Bridge... or AMD if you decide to switch.
As mentioned above, Haswell is going to be released this summer. It is socket 1150 so it is not compatible with motherboards designed for Sandy Bridge / Ivy Bridge. The increase in performance is unknown so far. I'm guessing 5% - 10% above Ivy Bridge (at the same clockspeed) since Intel is focusing more on lowering power consumption than increasing performance.
AMD is also expected to be releasing the Radeon HD 8xxx series if you have no preference for GPU brand.
Generally speaking, most games are limited by the GPU rather than the CPU. So if you plan on upgrading the GPU as well, you may want to do that 1st before upgrading the CPU. That way you can upgrade the CPU later.