Yes, D* does have a number of DD5.1 channels, I don't remember them exactly but I think it's the main HBO Showtime and Starz and a few PPV's.
One thing to consider with using your standalone Tivo with a D* STB, surfing will be VERY slow (I think). I tried doing this with a standalone ReplayTV box and it took forever to change a channel. The RPTV box had to send each digit of the channel you want to go to via the IR blaster, then the RPTV box had to buffer the data before showing it, so each channel change took like 3 to 4 seconds. Even just doing the jump up or down on channel took this long since it still send the exact channel number.
If you went with the D* Tivo box you eliminate that issue plus you'd be able to record the DD5.1 audio stream, something that I don't think you can do with your SA Tivo. You'd also get the dual tuners which is also a really nice feature. Just my two cents.