To make it easy for the thread starter:
Skies of Arcadia Legends
Evolution: Worlds
Lost Kingdoms (AKA Rune)
Lost Kingdoms II
Baulder's Gate: Dark Alliance (can be found in most offbeat stores for a mere $8)
Final Fantasy: Crystal Chronicles
Harvest Moon: A Wonderful Life
Animal CRACK-er, Crossing
Future Tactics: The Uprising
Medabots Infinity
Gladius
MegaMan X: Command Mission
Summoner: A Goddess Reborn (pretty damn good PS2 port, plus extras)
XMen Legends (not sure if this has been cancelled or not)
Yu Gi Oh!! ( I won't bother with the subtitle, simply because I hate this franchise)
Tales of Symphonia (due out in a few months)
Baten Katos (also due out in a few months)
Phantasy Star Online: Episodes I and II
Phantasy Star Online: Episode III C.A.R.D. Revolution
Pokemon Colesseum (well, it's sort of an RPG since it has more elements from the main GBA series)
Bascially pick through this selection here, read up on them, and pick up/rent whatever seems more attractive to you. And because you said you like hack-n-slash action such as Diablo, Hunter: The Reckoning isn't half-bad, though not quite an RPG.
Also, keep an eye out for any new GCN offerings from Square/Enix, as I'm SURE another FF and/or DragonQuest will make their way in due time.
-Official Ninja of PGC