Graphically, there has been a lot added to The Legacy of Ys: Books I & II over the years. The basic characters, backgrounds, monster, and the rest of the in-game stuff have stayed mostly the same. To add to the game, there are now beautiful cut-scenes and intros. These new scenes are on par with any of the recent DS games that you've seen. You might think that this would be distracting going from a beautiful intro to sprite characters, but on the small DS screen, all RPGs end up doing that, even the ones written today.
Now you know music is good when they release a CD of it. There are almost 50 releases of CDs from the various Ys games. More than 10 of those are from Books I & II. So you can guess that you're going to enjoy the background music throughout these games. The sound effects are quite good too. This is one of the few games that I recommend playing with the sound on, just because it's so pleasant.