The way I see it:
Alan Wake is not a big system seller.
Splinter Cell probably won't be a big system seller unless it comes up with the next big multiplayer game.
Gran Turismo is a huge system seller, when every GT has sold close to 10 million or more, only those who aren't really thinking about it would claim it's not a system seller.
Final Fantasy XIII is a huge system seller, and since it was developed for the Playstation and has a fanbase that consists of Playstation owners, it will push more PS3's than 360's.
God of War III is not a system seller. Just... no.
The Last Guardian probably isn't a system seller even though I think it should be.
Fable 3 is not a system seller when we've already had Fable 2, unless it does something revolutionary like Molyneux always claims the series will do.
Heavy Rain is not a system seller.
Sure, some of these games will give a slight boost to weekly sales, but we're talking 10k-20k. The big games like FFXIII and GT5 will boost sales 50k or more, with lasting effects. FFXIII might see that 50k spread across systems though.
And to answer the OP, the 360 has no system sellers until Natal.
Currently playing: Uncharted: Drake's Fortune, NBA2k11, Metal Gear Solid, Picross 3d