I know this has been some time, but in an effort to update the threads people come across when searching this (such as me), there are other options for both the CPU and the GPU.
First, the CPU. The Intel i5-680 3.6 GHz is the fastest 2 core processor you can use with the highest single core benchmark scores. However the i7-860S and i7-870S 4 core processors share the LGA1156 socket and work. These S variants are a 82 TDP and work in the 21.5" iMac. The i7-860S is a 2.53 GHz with a turbo boost of 3.46 GHz, but 4 cores. So while the single core benchmark scores average slightly lower than the i5, the multi-core scores are much better! The i7-860S is much easier to find and a lot more affordable than the i7-870S. The i7-870S is a 2.66 GHz with a turbo boost of 3.6 GHz. It can be hard to find and is over twice the cost of the 860S. Also, it is only very slightly faster. From looking at the Geekbench 4 tests, the score results are extremely close to just marginally faster… not really worth the much higher cost. An advantage of the i7 chipset is that you can use 32GB RAM, even with the 21.5” model!
Here’s my CPU benchmarks. First the i5-680 (3.6 GHz 2 core) -> https://browser.geekbench.com/v4/cpu/119... and then the i7-860S (2.53 GHz 4 core) -> https://browser.geekbench.com/v4/cpu/115...
For the GPU, the ATI Radeon HD 5670 (512 MB GDDR3) was the top 2010 GPU you can use. However, you can install the 2011 21.5 iMac's AMD Radeon HD 6770M (512 MB GDDR5) P/N 661-5945.
Here’s my OpenCL benchmarks. First the i5-680 with the ATI Radeon 5670 (512MB) -> https://browser.geekbench.com/v4/compute... and then the i7-860S with the AMD Radeon HD 6770M (512MB) -> https://browser.geekbench.com/v4/compute...
Just so this information can be seen, here’s the ‘About This Mac’, ‘Overview’ and ‘Displays’ from my 21.5” 2010 with the i7-860S CPU, 32GB RAM and the Radeon HD 6770M GPU.
I case there are any questions about the second 43” display with 3840x2160 resolution. I’ve got a 4K TV that wanted to also use as a secondary monitor, but a basic Mini DisplayPort to HDMI adapter could only feed 1080p. After some search, and a couple of products tested by trial and error, I found two that gave me this 3840x2160 resolution. Ones the Plugable brand: Active Mini DisplayPort to HDMI 2.0 Adapter, the other is the Cable Matters brand: Active Mini DisplayPort to HDMI Adapter. The image is outstanding, and higher resolution than I expected to get since all the specs I found list the max secondary display resolution of 2560x1600. The only drawback is that I have only been able to get 30Hz.