If you look at <a href="http://www.vgchartz.com/hwcomps.php?cons1=PS2®1=America&cons2=GC®2=America&cons3=XB®3=America&weekly=1">this chart</a> you'll notice that the large spikes are sales boosts in November/December for Christmas. Beyond holiday spikes many/most of the large shifts upward in the lines represent price-cuts of hardware or hardware finally becoming readily avaialbe. Most game releases are represented by the small bumpy "noise" in the graph; some games represent spikes but not that many.
The reason for this is that a game release is factored into the sales of systems months before and after the game is released; many people who are interested in Halo bought the system with Gears of War, others will have bought it with Forza 2, and even more will buy it with GTA.
Overall, I would not expect a dramatic shift in the performance of any systems because of the release of a handful of games.
