I've yet to see a program that will actually make use of that much RAM. All I can think of is running Photoshop and Parallels with Vista inside of it which is also running Photoshop. Therefore I doubt the large investment into a 4GB RAM stick is worthwhile, though it may be eventually.
I'd personally wait for the price to drop. Also, we may see custom sets of two 3GB sticks for full use of Intel's dual channel memory setup. Intel has two types of dual channel, and the most optimised version of it runs when the two memory chips are the same spec and same size (matched pairs). Thus unless you're actually needing more than 4GB of RAM you may see a performance hit due to use of non-matched pairs. Generally the more RAM the better, but again at 4GB you may not need anymore yet. Thus loss of optimised dual channel is likely to be more of an issue. In the original MacBook's when Apple said the limit was 2GB, but in reality you could use 3GB the use of non-matched pairs made sense since having more RAM available actually helped system performance. The same may not hold true for more than 4GB at this point in time.
