I have 8Gb of RAM in my iMac, and was just running over 20 apps (like iMovie, iPhoto, Pages, Numbers, Keynote, Safari, Chrome, App Store, iTunes, Games Centre...) and everything was still buttery smooth. And the RAM usage was just under 6Gb (and that was with other RAM caching also, so the actual use was around 5Gb). I still have another 2Gb before it clears the RAM cache, then another 1Gb before it compresses, then another 4Gb before it starts to Swap
The best part: I almost always only use one program at a time8Gb is super over-kill
I'd say 4Gb of RAM is enough for the basic user, even if they wish to have it for 3-5 years. 8Gb for a more advanced user, that wishes to keep it for 4-6 years, then 16Gb of RAM for the advanced user that runs a lot at once (and maybe VM's too) and wants to keep it for 4-6 years.
iMac doesn't need to share 1 GB of ram to the graphics chip