Technically you can replace the Graphics unit with another, but the compatibility is not tested for larger-memory card. However, keep in mind that you will have to use the same board as in your current iMac, which makes the mission very very hard. If you find your system slow, think about replacing the HDD with SSD. This will increase App launch speed significantly.
I will suggest to remove the other end of the socket which is connected to the display panel. You can never imagine how difficult it will be to plug in on the main body, unless you know well about it.
