Yes, beware the synthetic benchmarks (non real-world usage) of dealers selling ram. OWC's 'Adobe Photoshop CS5 medium action test' shows nominal difference between 12 and 16GB of ram in their test measuring in seconds how long it took to run a 'custom' script of 45 common memory-intensive processes on a photograph using Adobe Photoshop CS5, designed, quote, "to represent a system-straining photo editing session of a graphic artist on an iMac machine such as with multiple images, images with multiple layers, or large images such as retail signage, movie posters, event signage, promotional banners and more. These same type of results can be expected from video production suites and other 64 bit applications."
4GB........6GB........8GB........12GB........16GB
556.18....476.18.....423.55.....325.14.....310.61
But getting back to real-world usage which TMRaven and infidel69 have discussed, if you are confident about working inside your iMac I agree also with the suggestion that the SSD drive will make for a vastly more noticeable difference. Though it doesn't get mentioned in reviews very often if you do a search you'll find OWC's SSD drive benchmarks well, or at least in the review I read, and they do a fitting service. I wasn't able to find any tests of reliability, though, which is a sticking point with me.