RolStoppable has it exactly right.
Most people seem to ignore the fact that GTA is a huge casual gamer game. If MS knows what they are doing they will drop the price of their console by $100 in time for its release. If people had to choose $300 for a 360 or $600 for a PS3 there would be no discussion, no thought, they would buy the 360 version. As it is, I expect the 360 to get a bigger sales boost than the PS3, and I expect the sales of GTA4 to be much lower than the previous versions overall.
Since that's already been predicted I will add another one. I predict that the sales ratio of Core:Premium X360's will be much higher than average. If the 20GB PS3 was still around, I would predict the same thin for it.
Help! I'm stuck in a forum signature!
