I believe that Wii Fit will trump both versions of GTA IV this year. Only because it is more accessible to everyone (and their grandmother) whereas GTA IV is rated M (not like that's stopping anyone from purchasing the game for their little bros =P).
Imagine how many copies Wii Fit will sell around launch date and I imagine it would have tremendous legs leading up to Christmas and onwards. SSBB was released just a couple months ago and has already sold more than 3 million copies here in America alone so Wii Fit shouldn't have any problems surpassing that amount. You also have to take into consideration that the software is being sold in three different regions compared to GTA IV currently on sale at two.
Proud Owner of all 3 current-gen systems
Currently Playing:
SSBB, Fire Emblem: Radiant Dawn, MKWii, GTA IV (360), MGS4, DQIV, Jeanne d'Arc, FFVII: Crisis Core, Chrono Trigger (PSX)
Mario Kart FC: 4897-6731-4375
SSBB FC: 1504-5406-8515
Wii Friends Code: 1020-2726-1535-3964
XBL: PhatLaosBoi
PSN: PhatLaosBoi







