just wait for haswell, yes it makes SHogun 2 better but if you need a few extra FPS disable Blood Pack DLC, that has a fairly noticeable CPU hit as its rendered in the CPU test
Sandy to Ivy is a 5-10% gain with better power consumption aka its the TEST for 22nm cpus,
Haswell will be the real deal Intel has a Tick Tock schedule
i7 920 = 45nm Tick
i7 930 = 32nm Tock
i5 2500k = 32nm Tick
i5 3570k = 22nm Tock
Haswell = 22nm Tick
Haswell Successor = xxnm Tock (to lazy to look up the info)
yes with about 4.4 you can get 30 + at all times with Ivy and a good GPU,
but Haswell should be another 20% gain
Nehalem to Sandybridge 20%
Sandybridge to Ivy 0-10%
Ivy to Haswell 20% give or take
Haswell and GTX 700 and HD 8000 series gpus will all launch before Rome II, so i recommend just holding out drop shadows 1 notch, and turn off blood pack if you have it and try and deal with it as best you can.
if you had a first gen i5 aka 750 or something id say upgrade but your already on Sandy and only a few games benefit from high clocks overclocking achieves
Bad Company 2
SKyrim
Shogun 2
off the top of my head anyway. otherwise it takes crossfire / multi gpu to see this kick into bigger effect in more titles, in which case memory bandwidth via the IMC on the cpu becomes more important in which case Sandy is just peachy there as well