If I may interject here amidst all of the great suggestions, you sound very much like myself when I am about to make a big purchase. Going back and forth...what's better, should I do this, or should I do that.
I don't have a 27" iMac, yet, but it is within a couple of months. After trying them out at the Apple store and witnessing for myself how many programs you can open right along the Dock one after another, and how FAST they load, I am pretty convinced that the 27" (i7 or i5) would be a great choice. They don't carry the 27" i7 in the store, but what I saw the i5 do was stunning.
Personally, I plan on getting the 27" i7 with basic RAM, and then upgrading that to the full 16GB through an online supplier. I figure that it's capabilities, purchasing AppleCare of course, will serve my purposes for at least a few years if not more.
If you like the 27" quad core, get it. I honestly think that going overboard a bit is better than running out of room, speed, and capability for $500 or $600 less...