Your system uses a MXM II slot video card. As to whats available you are limited to the ones Apple has used in their system and physically fits (they also have custom firmware).
21.5" iMac Mid 2011 Video cards:
- AMD Radeon HD 6750M, 512MB - Apple P/N 661-5944
- AMD Radeon HD 6770M, 512MB - Apple P/N 661-5945
What you are asking for is not workable in your system these cards are larger and have a different heatsink
27" iMac Mid 2011 Video cards:
- AMD Radeon HD 6770, 512MB - Apple P/N 661-5967
- AMD Radeon HD 6970, 1 GB - Apple P/N 661-5968
- AMD Radeon HD 6970, 2 GB - Apple P/N 661-5969
Given the costs and work (risks) I think you'll find it cheaper looking for a used 27" Mid 2011 model which has the better card or even get one that doesn't and upgrade it if you really need this better card.
Frankly, unless you are playing video games or editing vids or pics you won't see much benefit without having the larger screen.
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Your systems specs: iMac 21.5" 2.5 GHz i5 (Mid-2011)