Just to throw a wrench in everything:
Yeah, the DS card can store more data, but also keep in mind that the Pokemon games have also had secondary backups at least since R/S. Actually, it wasn't even four weeks ago that I had my Emerald save file corrupt on me during a bad trade with XD (I'm guessing the link cable shook loose or something) and had to revert to the backup file. In other words, any extra amount of space being used on the save files will be increased because of the backup.
But with all that said... I do think the 650-750 figure being thrown around is really feasible. I'm sure they want to avoid Animal Crossing: Wild World-level saving/loading times, though, so don't expect 1500 spots or anything. (I know, Pokemon Box. But that didn't have to keep track of other things like items, location saved at, which events in the game have been completed and which haven't, etc.)