You could try an i7 on a 1156 socket, like the i7-870 (http://ark.intel.com/products/41315/Intel-Core-i7-870-Processor-8M-Cache-2_93-GHz)
But this CPU was originally only available in the 27" iMac.
This processor also has a higher TDP of 95 watt. Your i5 uses max 73 Watt.
Using the i7 in your 21.5" might be problematic concerning the heat dissipation.
There are a lot of 45 W or lower i7 CPU's, but i think they are much slower then the i5 you have now.
The other bunch of i7s is 82 W or higher, which may cause problems in your iMac. Even the 82 W i7-860S is slower than your i5.
I think if you want a faster system...you need to buy a new iMac.