The salesperson was probably trying to tell you that a 768 display will not accept a 1080p signal from a HD player. This is correct (I know because my 768 display will not accept a 1080p upscaled signal from my Lumagen processor).
Therefore, if you want the most future-proof display that will get the most out of the HD players then a display that accepts 1080p is the way to go - but note that most 1080 displays still won't accept 1080p either, only 1080i.
But the difference in picture quality may not be great, depending on the size of the display, and 768 may be what you're looking for. Personally, I'm looking for a display that will accept 1080p, have a built in digital tuner and multisystem analogue tuner, and look really cool. Haven't found one yet.