Majority might have bought GT3 for the PS2 to experience GT in PS2 graphics. For GT4, it was released on the PS2 too, thus some might think they don't need to buy another GT again for the PS2 since they should almost be the same experience. GT4 might have appealed to only hardcore and racing fans while casual racing fans might not have bought it.
GT5 is the first GT on PS3 and in HD, thus demand will be very significantly much much higher. I can say safely that it will at least sell 3 million in the US alone.
Racing is a very broad genre and almost everyone have some inclination towards racing games. I myself am a more RPG/Strategy/Platform/Action kind of gamer but I will definitely buy GT5. This will be my first GT. God of War 3 is a possibility too but I am still not sure about buying it. But I am already all hyped for GT5.







