Hmmm.
I think God of War II is being undertracked, I read on IGN that it outsold the original.
It's looking more likely that Gran Turismo 5 will release in 2009, that will decimate God of War III in sales.
Final Fantasy XIII is from a much larger series, that will outsell God of War III.
So I'm going to say it will be third:
1) Gran Turismo 5 - 2.4 million opening week WW
2) Final Fantasy XIII - 1.7 million opening week WW
3) God of War III - 1.2 million opening week WW
4) Killzone 2 - 900k opening week WW
5) Call of Duty: Modern Warfare 2 (PS3 Version only) - 800k opening week WW.
Bold predictions, I know.