If you can return the MacBook Air to Best Buy without any penalty, buy one now but don't open it. Wait to see if Apple announces anything better in the keynote in three days.
If Apple does release something new that you would prefer instead of the MacBook Air, return that MacBook Air to Best Buy for a full refund. If not, or you think the price difference for the new laptop is not worth it, you already have your MacBook Air and will not have to worry about being able to buy one before it is out of stock forever.