Trying to upgrade iMac A1312

Sadly your options are limited to what Apple used in this series 2011 27” iMacs depending on what you currently have there might not be an option or not enough of an improvement. As an example jumping between the i5 to i7 only gets you a bit more multi-core performance which depending on the Apps you’re using won’t be very noticeable.

For most maxing the RAM and installing. SSD drive as the boot drive holding your Apps and having a fair amount of free space so the system has some elbow room for virtual RAM, caching and some apps use scratch space. A smaller SSD (less than a TB) should have 1/3 free and a larger 1/4 will do.

As far as Graphics again your options are limited! This series uses a MXM board which is no longer being made and Apples OS drivers only support what they used at best a newer version of the board will use compatibility mode so you don’t get all of the benefits the newer Gen boards offer and to this the cooling rig needs to fit the board and Apples boards also have custom firmware which needs alteration to work correctly.