My iMac is now about 3 years old (the last of the "square" 3x4 monitors). Is there really that much of a performance boost? I seem to be able to play Diablo 3 well enough. And even with Apple's excellent transition software, I don't relish to down time it takes to transfer everything.
The geekbench scores for the 2011 i5 and i7 27" iMac are about 2x and 3x the score respectively for your 2009 iMac.
While geekbench scores do not directly translate into practical performance (it depends on how you use your system) they do speak to the relative performance capabilities of the systems they measure.
But, if you are happy with your system, then don't worry about it. There is always something faster out there.