If you're having FFVII problems, just find the "ultima edition" for PC. Once I put my copy on, I didn't even need the discs again, and it took less than 200 mb total space.
FF V is THE greatest FF game, period. I've been playing them since I managed to get a hand on the original FF for my NES, and out of all the games (minus XI and XII, which I haven't played but I don't really count because the break the mold) I find V to be the best.
Really, since GBA games DO work on the DS, your opportunities for some good old fashioned dungeon crawling are wide open. Dungeons and Dragons: Eye of the Beholder is a classic, though the GBA version's combat system and switch to version 3 rules feels odd in such an old school game. Then there's DemiKids, kind of a cross between Pokemon and Shen Megame Tensei, kinda weird, but worth trying. Golden Sun, obviously, both are well known and easily available, though the gameplay is for the most part extemely simple until you've collected a massive number of the djinn, at which point it gets crazy. Lunar Legend- sucks compared to the original Sega CD releases, especially with the loss of the videos, but the gameplay is easy to get a handle on, and the story is fairly good as well.
On the DS itself, they really haven't released that large a number of RPGs yet, but when combined with the massive numbers of GBA cartridges available, and the pending releases, there shouldn't be any problems.