It'll definitely cross a million. Ratchet and Clank is a reasonably popular series, and really, if people bought ToD over Uncharted, Modern Warfare and Assassin's Creed (or bought all four, like I did- well, I didn't actually buy AC, it was a gift), then they would buy A Crack in Time instead of or alongside Uncharted 2, Modern Warfare 2, and Assassin's Creed 2 (which I don't expect to sell as well as the original).
The PS3's userbase is larger now, and yes, userbase does matter, when you are talking about a tiny userbase like the 6 million or so the PS3 had at Tools of Destruction's release versus the 26 or so million that it will have at Crack in Time's release. Back in 2007, the PS3 sold abysmally every week, and was considered a failure console with no games, and a ridiculously high price tag. Also, ToD and Q4B built up an R&C fanbase on PS3. A PS3 price cut should happen around Crack in Time's launch. Not to mention, Crack in Time will be longer, have more game modes (including online) and will have DR. NEFARIOUS!*
*For those of you not awesome enough to have played R&C 3, Dr. Nefarious is the best Ratchet and Clank villain, and definitely one of the best villains of all time. R&C3 is the most critically acclaimed AND best selling Ratchet and Clank.
It has a shot at 2 million, but 1 million is guaranteed.
(Former) Lead Moderator and (Eternal) VGC Detective 
