I think it's a better strategy to buy the computer before the new OS comes out. If you buy after the new OS is released, it likely won't support running the old OS version. That means you're stuck with the new OS, which often has major bugs that need to be worked out in the first couple of OS updates - a process that can take months. If you buy it with the old OS installed you won't need to upgrade until the new version is solid, and you'll always be able to fall back on the old version if there are problems.
As to the Macbook specifically, probably wait at least until WWDC next month to see if there's a processor speed bump coming. Also there may be a back-to-school promo this summer that could mean a free iPod with computer purchase.