Best CPU and GPU for my iMac 21.5" late 2011

Hi guys! What will be the best CPU and GPU for my iMac? I am considering all variants even DIY stuff and tested experimental setups. As far as I know Intel i7-2600S is best CPU for this machine but what about GPU? Also, I did not mention this in the title but what will be the maximum amount of ram supported? Does it also depends on CPU?

Btw, specs:

Intel Core i5 2.5 Ghz

AMD Radeon HD6750M

4 GB 1333 Mhz DDR3