The projector has a scaler built-in as do all HDTVs. DVD players and receivers are now also incorporating scalers (usually described as 'upconversion' which is a generic term that actually means different things in different contexts).
Unless you spend big bucks on an excellent outboard scaler/video processor that is definitely better than the one in the PJ you aren't likely to notice any significant improvement.
I'd get the PJ and get everything set up and then consider an outboard scaler only if you feel things aren't quite as good as you'd like.