I don't think many people will go to a store to buy a 360 to play Gears of War or Halo 3 anymore. They might pick it up with some other titles or it could be one of the titles they're interested in and make them buy a 360, but those titles are not the main reason anymore for people to pick up a 360. If it was then they would have done so by now. Of course the complete library of games will play a factor in purchase decisions of consumers, but if you think 2 or 3 games will completely change the outcome of the console war then you're wrong. Well, I can see 1 game which has been that influential, but it is called Wii Sports and is neither on Xbox 360 or PS3.
Edit: I'll also answer your next post here as well. You didn't mention talking about exclusives in your post so I assume you say all games released on PS3 before GTA IV fell short of being system sellers. I didn't put the bar that high to be called a system seller, you did that yourself. But from this list:
GT5
MGS4
FF13
Resistance 2
Killzone 2
LBP
Motorstorm 2
Tekken 6
etc.
Everything below FF13 is uncertain to be system seller by that standard and 2 of the 3 will most likely not be released in 2008.












