Yeah selling 32k seats should be no trouble whatsoever for Premiership teams except maybe someone like Wigan who have shot up from the lower leagues in a short space of time and where football isn't the main sport in the town; Saints are in League One and they took 44k to Wembley for the Johnston's Paint final (would have taken more if they could).
OK so that was a final, but it's not like Stoke and Bolton have had bulging trophy cabinets in recent years, semifinal of FA cup should be a pretty big deal for them. Maybe a better example would be something like Pompey vs West Brom, that had 83.5k attendance, presumably they sold out most of their allowance.