That is different since the source is interlaced if 1080i - it is best to leave it native - if the source was 1080i let the projector scale it to 480p. If it was 720p ditto. But sometimes you do not have the choice. I prefer 720P because vertical resolution scales better - you may like 1080i because it has more horizontal color resolution.
But stick to 480i for SD channels - the SP4805 is much better than your cable box at deinterlacing - and you do not want to upconvert with a bad scaler only to downconvert. This does mean you need the latest firmware to autosync as you channel surf.