Apple iMac "Core i5" 2.7 27-Inch (Mid-2011) Specs Identifiers: Mid-2011 - MC813LL/A - iMac12,2 - A1312 - 2429
Details: *Officially, Apple provides no information about whether or not the processor in this model can be upgraded and the company does not intend for one to upgrade the processor either. However, this processor supports an LGA 1155 (H2) socket and it is possible to upgrade the processor to some extent.
You should have no problem upgrading to this, but I would stay with a known Apple version of it:
Processor Speed: 3.4 GHz Processor Type: Core i7 (I7-2600)
Details: This custom configuration is powered by a 32 nm, 64-bit Intel Core i7 "Sandy Bridge" processor with quad cores (four independent processor cores on a single silicon chip). Each core has a dedicated 256k level 2 cache and all processor cores share a single 8 MB level 3 cache. This "Core i7" processor supports Hyper-Threading (it has two threads per core for a total of eight threads for the system, effectively four real cores and four virtual ones) as well as Turbo Boost 2.0 (the system can dynamically increase the processor clockspeed to 3.8 GHz when tasks demand).