I know I’m almost seven years late, but maybe someone else will find this helpful.
For a 21.5” Mid-2010 iMac, the best CPU you can upgrade to is a Core i5-680 (https://ark.intel.com/content/www/us/en/...). You can get one for under $25 on eBay right now. I don’t believe it’s possible to upgrade to an i7 as all the i7 processors I can find that fit in this iMac’s CPU socket have a higher TDP. I think (but am not even close to certain) that if you want a quad-core CPU, a Xeon L3426 (https://ark.intel.com/content/www/us/en/...) might work; please don’t quote me on this though.
To upgrade the CPU, you’ll want to follow the iFixit guide: iMac Intel 21.5" EMC 2389 CPU Replacement. I don’t know what ACP was saying about replacing the entire logic board in order to upgrade the processor—that’s just plain wrong.
Regarding your question about the graphics card: I, too, am trying to find out what the best GPU upgrade is for a 21.5” Mid-2010 iMac. So far, the one you currently have is the best one that I know of that works. If anyone else does know of a better GPU for this iMac, I’d love to hear about it. I’ll also post an update if I find anything.
Here’s the iFixit guide for upgrading the GPU in a 21.5” Mid-2010 iMac: iMac Intel 21.5" EMC 2389 GPU Card Replacement.
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