FWIW, I agree with Moth3r that in nearly all cases, 192k AC3 should be just fine for 2.0 tracks. I tend to encode all "alternate" tracks (dubs, commentary, etc) at 192k and think it's perfectly reasonable. For "main" tracks, though, I throw just a hair more bitrate at it (no objective reason for this)--if I don't do lossless, then I do 224k AC3 for "main" stereo tracks. Those are the only two bitrates I've ever seen on mainstream commercial discs for DD stereo tracks. Mastering differences account for most if not all of the differences you might hear between a lossy DVD tracks and the equivalent lossless Blu-ray track.

You can save a lot of space, without sacrificing quality, by encoding mono as a single channel. i.e. on Laserdiscs and many other media, mono tracks are encoded as two identical channels for left and right. If you encode it as a single channel, you can get away with half the bitrate--96k or 112k using my methods above. Some people don't like single-channel mono tracks because of how some receivers don't upmix them the way they want, but you can't please everyone.