There seems to be two different arguments here. Will Modern Warfare 2 out do Halo 3's opening week and the other will Modern Warfare 2 out sell Halo 3's lifetime sales.
If you are going strictly on the Xbox 360 platform for sales of MW2 and H3 for the opening week, no I don't see it having a chance to outdo Halo 3. If you combine PS3 and Xbox 360 sales for the opening week, maybe. But I don't think it is set in stone thing.
On the debate of weather it will oustell Halo 3's lifetime sales, I don't see it happening strictly on the Xbox 360 platform. Halo 3 will be going down in price, bundled and Halo 3: ODST an expansion is coming out for it which will boost the sales of Halo 3. If you combine X360 and PS3 sales yes it could. Both have a bigger userbase than when Halo 3 launched and Halo 3 is a Xbox 360 exclusive so it doesn't have the chance to have that userbase.








