Graphics can have an impact. One famous example is Myst. Not much of a game, but a huge seller for the PC. On the other hand, you don't need 1080p to play Tetris. So sometimes good graphics sell a game, other times good game play sells it. All the rest of the time, it's good marketing of a bad game.
At one time, the graphics power of a system was a major limiting factor of what games could be made. But after the last generation, it's much less of an issue. Sure, there are going to be some games that will not be possible on the Wii due to it's lack of power. And vice versa for the other two not having the unique interface of the Wii. And I'm not talking about HD or number of polygons. I'm talking game types/modes/features. A first person shooting game is a first person shooting game. 480p or 1080p doesn't make much of a difference for many. But if one controls better or the other offers realistic physics that matter, those could be the factors that count. At this point, however, developers are still grappling with just getting their games out the door. So it's unlikely that we'll see much innovation before the end of the year.
We could argue all the merits for an against prettier graphics until the end of time. (And at this rate, we probably will.) But at the heart of it all is good games. All the systems will have them. May the best one win.
Numbers are like people. Torture them enough and you can get them to say anything you want.
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