Do you mean Dolby Digital? I'm not sure what kind of digital output your TV has, I've never seen a digital output on a TV...
If you're talking about inputing AC3 sound through the digital in on the GTXP, having software decode it and playing it through 4 speakers (like you can do with PowerDVD), I'm almost sure it's impossible.
If you're inputing a stereo sound, it won't really matter, although theoretically digital should be clearer. And yes, the GTXP will play it to the 4 speakers, but it'll just be a stereo sound playing with the front speakers and reproduced on the back speakers.