I will admit that I haven't been a fan of the MGS series and haven't even bought a single one of the games in it. However, after seeing the play thru that Kojima did in Japan, I know that I will be buying this game. Do I think it will do Halo 3 numbers? My honest answer to that is HEEEELLLLL NO. The only way it could is if they make it into possibly the best game ever made and people are going nuts just to get the game.
GT5 though, in the long run it could beat Halo 3 numbers, but it will be close. With a title like GT, it usually has very long legs and sells very well all over the world as it's arguably the best racing sim ever made.
But I do know that GTA4 will beat Halo 3 numbers. The hype for this game makes Halo 3 look like it didn't even get a single ad.
Oh, but I will make this prediction.. MGS4 and GT5 will outsell Halo 3..... in Japan.

Get your Portable ID!
My pokemon brings all the nerds to the yard. And they're like, "You wanna trade cards?" Damn right, I wanna trade cards. I'll trade this, but not my charizard.