First, Amazon is giving all pre-orderers a $10 voucher for an immediate download of Max Payne 2, which just so happens to cost $10 on Amazon, so that's free. Amazon will knock down the price for both L.A. Noire and GTA IV to $4.99 up until April 15. You'll be able to grab those gratis with another $10 voucher if you want, or spend it on other Rockstar wares.
On top of the free games, a pre-order will also grant you several pieces of in-game content: a classic skin pack for multiplayer that includes characters from past games; a multiplayer weapons pack; increased painkiller capacity; and a "Max Payne Retro Avatar."
Update: Whoops, as you probably noticed, it's April 18. Amazon just got in touch to say they are extending the deal on GTA IV and L.A. Noire until April 20.