Forza 2 IIRC sold around 3+ million, and was heavily bundled. I remember getting it with the 360 when I bought the system, and I never even opened the game yet.
That said...sequels rarely sell better when a previous version has already been released for the system. And F3 probably won't be bundled like F2 was.
Now GT5 has historically been a monster success and always sold very well. GT5: Prologue even sold about 3 million copies and it was merely (as the title suggests) a prologue. How much better will GT5 sell than F3?
"I don't have time to play video games anymore, but if I did, I would definitely choose the PlayStation 3 instead of the 360" - President Barack Obama