You wouldn't want to buy a product when the line-up is going to be refreshed in another week. So go to MacRumors: Buyer's Guide to check out!
If you happened to have bought Apple's old line-up product, Apple has a price protection for 14 days after the purchase. So you can bring it back to Apple Store for upgrade or change for pretty much free (unless the new line-up is more expensive that the one that you have bought). Terms and conditions might vary with different countries.
MacRumors: Buyer's Guide