Dan, can you receive packages from Amazon? That would be the place to start pricing it out.
I'm gathering some info for a friend on this right now and happened to have these two articles on my clipboard:
http://support.apple.com/kb/HT4621
http://support.apple.com/kb/HT3918
I think this is a pretty easy operation, but it just depends on you. Depending on the year and model you may want to double-check on compatibility between the iMac and the specs of the memrory you purchase.
When I upgraded I was surprised at the improvement; I guess anyone would agree that upgrading the RAM will improve your experience across the board, including games; it certainly would be an easier/cheaper place to start than upgrading your graphics card.