The justices in question are Justice Stevens and Justice Ginsburg, which means that the President wouldn't have much of a chance to change the direction of the Court.
It will be interesting to see whom the President taps for his next Supreme Court vacancy. If he has the opportunity to choose a justice while the Congress is still dominated by Democrats, I'd expect someone like a law professor -- or an "intellectual" former senator who could benefit from the traditional courtesy the Senate extends to its members.
My guess is that Obama is looking for someone to be an intellectual counterweight to Justice Scalia. While Justice Sotomayor may be reliably liberal in her rulings (and her status as the first Hispanic justice calculated to appeal to a growing demographic) she isn't that. The law professor in Obama will want a left-wing "intellect" as his second appointment -- perhaps someone in the mold of Cass Sunstein (on the law professor side) or Hillary Clinton (on the former senator side) -- if she'd take it.