16GB or 8GB on iMac.

Those 4GB modules are pretty expensive. The most economical choice is to buy the iMac with the stock 4GB of RAM (2x2GB). Use it that way initially. For everything you mentioned except Final Cut Pro, 4GB should be sufficient. For Final Cut Pro, the RAM you need depends on what you do with it. If you need more RAM, you can get after-market RAM and install it yourself for less. For example, from Mac-oriented vendor OWC...

Another 2x2G to add to what you have for 8GB total costs $113

http://eshop.macsales.com/item/Other%20World%20Computing/8566DDR3S4GP/

Even replacing the stock 2x2GB modules with 4x4GB costs $900 (and you get to keep the 2x2GB)

http://eshop.macsales.com/item/Other%20World%20Computing/8566DDR3S16S/

So therefore, depending on where you live and the cost of purchasing RAM, you may actually save money by starting with the lowest cost option and upgrading up from there.

I ordered my i7 with 2TB HDD and 16GB ram. I felt it was worth it. Why have a fast processor and slow it down with inadequate ram? Those who think the added options are too expensive are those who can't afford them. I want a computer built for the next five years for whatever evolves. True, you can always add ram later and it MAY be cheaper. Ram has increased in price as of late. Finding compatible ram isn't always easy. I have found some which didn't work in my Macbook Pro. With my i7, I ordered all the upgrades including the ram which is now covered with Applecare for the next three years. I have fast cars and want a fast computer. Get your computer fast and watch all the other ones in your rear view mirror.

zoz

With Mac OS X, there is always some benefit from more RAM. With the current iMacs, 4GB is already pretty good. Going from 4GB to 8GB will have a bigger impact than going from 8GB to 16GB.

That will end up costing you $600. If you get it with the stock (2x2GB) and buy 2x4GB to install yourself, it will probably cost you significantly less than $600, and you will have a total of 12GB in the four slots.

Tokyo, I wrestled with the same question and was leaning toward Kenichi's suggested path to go with 12 GB of RAM. If your use is reasonably basic today, I'm sure the cheapest solution of 8 GB total (with four 2GB sticks) will be fine. !2 GB is better if you have the money and of course 16 GB may only show its worth when the Pro Apps really take advantage of the new features of Snow Leopard and load up all your processors and RAM to speed you along. For all your other day-to-day work, 4 to 8 GB is likely fine. So the more memory you install, the more you are ready for the future... but it has to be reasonable for you to balance that with the cost of course so you be the judge.

My wife finally said to go for the 16 GB option but that's for a system I don't expect to upgrade for five to six years. Some people prefer to stay closer to the mainstream (4 to 8 GB) and plan to upgrade their computer sooner because they'll want the newer features we don't have today (Blu-Ray, USB 3.o or that , 10 gigabit Ethernet, octo-core processors, etc.