You'd have to do some hardcore rendering or editing to tap out 6GB of memory. You likely won't see much if any difference by going over 6GB. Though, you could always get 6gb now and add more later if it's not up to par for you.
Now that CS4 has dynamic linking, it's possible that one would have AE, PPro, Encore and Soundbooth all open simultaneously + any background tasks and the O.S. Would all these apps open at the same time exceed 6GB? Someone with more knowledge on CS4 care to comment?
Yes, multiple Adobe programs definitely have the ability to make use of more than 6GB of ram, which is why if you are using those alot you should opt for 12GB, but unless you are using those quite often the performance increase in those programs wont be worth the premium for the extra memory. But if you already bought CS4 you can probably afford another 100 for the extra memory.