The 360 will continue to sell well over the entirety of 2007 and nothing will be the online replacement for Gears until Halo 3 does hit store shelves. You will get some great titles all across the board in different genres but they aren't focused as an online competitive shooter such as Gears of War ,or Halo 3. The closest that will show up is GRAW 2 which is going to be great for the fans of the GRAW series but won't do anything to bring down Gears. Mark my words, the #1 Xbox Live title will be Gears of War until the end of 2007.
Gears of war is not only the best third person shooter but the best game ive played in a long time since Halo and GTA 3. I really think it could still be amungst the upcoming next ge games in 2007. Just to name a few Halo 3, GTA 4, GRAW 2 and Kill Zone 2 are all great titles. But I think if GOW makes a sequence it will suck compared to the first so it probably wont hang with compitition.

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