Ordering a 21.5" iMac with a different graphics card
No you are correct there are no upgrade options available for the 21.5" iMacs (Mid-2011). Yes, you are stuck with what comes in each of the two 21.5" models available. Your eyes are not deceiving you on the iMac Specs, what you see is what you get no exceptions. 

You graphics card options are:


Buy the 21.5-inch: 2.5GHz model and get the AMD Radeon HD 6750M with 512MB
Buy the 21.5-inch: 2.7GHz model and get the AMD Radeon HD 6770M with 512MB


According to MacWorld's article / review "21.5- and 27-inch iMacs (Mid 2011)"


  


So in reality you have two options, and its all in which machine you buy. If all you want is a graphics boot, your going to have to pay the premium to upgrade all the other components too, unfortunately there is no way around it, other than letting apple know you want options, give them some feedback stating so.