Psst. Don't listen to Josh. Sandy Bridge uses a 32nm manufacturing process vs. the 65nm or 45nm in the core 2 duos. Smaller feature size translates to less heat generation, less power usage, and higher max clock speeds. In addition there are various other architecture tweaks.
Are you shopping for a macbook air? The sandy bridge version will be far more powerful than the current core 2 duo (70%+ increase in speed on some processor intensive tasks) and should have significantly improved battery life as well. Whether or not you need that is another question of course. At the very least I would wait for the new version to come out just so they will drop the price on the old one.