They dont use different sockets, they both use the LGA 1155 socket, so AppleGeek911 is totally wrong, and since your current cpu is a 3rd gen cpu, downgrading to a 2nd gen cpu probally would be just fine, some OEM Motherboards have the bios set for only some cpu, if you have an aftermarket Mobo it is more then likely on the cpu list for the bios, BOTH ARE SOCKET H2 LGA 1155, AND THE I5 2500 WILL WORK JUST FINE, i also have the i3 3220, and just bought a 3470s i5 from ebay for 90 euros, and i almost bought the i5 2500 or 2500k just like you, and yes even though the i3 3220 supports ddr3 1600 and the i5 2500 does not someone correct me if im wrong here, yes ddr3 1600 will work with the i5 2500 but i believe it wont run at full speed i think the 2500 cpu memory controller is for DDR3 1066/1333 as where the i3 i5 i7 (Third Generation) has support for 1333/1600....Hope this helps, it would be upgrading from a dual core to a quad core, and i would totally think it would be worth it, the i5 2500 boasts an impressive 3.30 GHZ, and the i3 3220 is 3.30 ghz, the i5 2500k would be the best , or the i5 3570k would be the Absolute best, both are "K" Models and can be overclocked very well.
http://www.intel.com/content/www/us/en/support/processors/000006050.html Proof is in the pudding