I bought a 2GB version and ran into a store to buy 4GB. Is about the same price as the online option from Apple, BUT you still own the old 2*1GB

So in theory I could sell it on eBay or something.
Azureus/Vuze are huge wasters of RAM, also Safari can grow up to 500M at least for me. So there is 1GB gone only for those guys. With my current apps running the memory consumption is:
Safari: 451M (3 open tabs with no special pages)
Finder: 113M
NetNewsWire: 87M
Skype: 80M
iTunes: 72M
Mail: 62M
Adium: 42M
Ical&1Passwd: 21M
All in all the current usage is Active+Wired=2.1GB
On my iMac At first I only had 2 GB, then upgraded to 3GB until the prices went down and just recently upgraded to 4GB. The upgrade from 2GB to 3GB was a hell of a difference. The upgrade to 4GB was nice when running heavy stuff like lightroom, PS, vmware.
Upgrading RAM is most of the times the easiest way to get a felt better performance. Because Applications that are not in use by you can be swapped out (written to harddisk). When you then switch from the front most to another application it might be a little unresponsive for some few seconds.
My recommendation: If you want your MacBook quickly buy the 2GB Stock and upgrade when you feel the need for it. Upgrading unsymetrically to 3GB is not such a bad option as people will tell you it is. But it allows you to explore how much RAM you really need and buy cheaper later.