I've seen at least 5 different people now say that with there 2500k on Guild Wars 2 were only getting 35-45 FPS.
Some were saying that there was a bug in the beta making it more cpu dependent but still.
Get the i5 3570k, no need to go bigger if you only want to game. Plus it OC's like a beast!
The reason why I say go Ivy instead of Sandy is because it is the latest technology enabling you to use PCI-e 3.0 (it currently does not have a big influence in performance but that might change within the next year, then you will be kicking yourself for saving $20 by going back to old technology).
I'd be more worried about your graphics card choice (s) and PSU in a nice case with plenty of airflow than the CPU.
Frankly the IVB cpu does not overclock well and thermally ramps quickly ... so its probably a better bet to get a SB CPU (or not push the OC much on the IVB) and moderately overclock that without having to worry so much about the heat.
A good gaming rig means spending more money on the graphics than the cpu.
Then of courese a nice big monitor goes without saying.
A 2600K looks a good bet and perhaps a 7970 ... or a GTX 6 series card if you can get one ... they are rumoured to exist.
Ivy Bridge is VERY over rated, the old sandy bridge is better, 2 reasons.
cost for performance
heat when overclocking.
get a 2500k top chip spend the extra on a better GPU which WILL effect your gaming and improve it NOT your CPU. that just makes multi tasking faster and in games its not needed to multi task, thats the GPU's job most of all.