Fleetmac said almost everything I could.
1) If Canada in in I actually feel Philadelphia would remain the capital, as it's a bit closer to this newfound center of the expanded USA. However, if the USA grabbed East and possibly even West Florida alongside Canada, the Potomac area remains the best center area for a compromise on the capital. Go figure.
2) I like the idea of New York as capital. It is the biggest city, which is always a plus as capital. It's also an organic city compared to Washington (even if to its credit Washington is finally, if at a snail's pace, becoming a city that lives on past government), and a wonderful natural port and city site as the east coast's best harbor. And it's so huge that becoming capital means that at least the locals will be used to a crowded metro area as in reality, so what's even
more people, I figure.
3) That said... I can only see it happening in a USA that happened to lose the entire south, though - be it a split in 1786 (when the Essex Junto tried to split the North off) or 1861 (the Civil War).