Ooh, I'd reccomend the whole Xenosaga series if you have only played the first. The second is rather on the 'meh' side of things, but the third is really quite good.
I'd highly recommend Tales of the Abyss that just came out for Ps2 - I'm a BIG fan of the Tales series, and like them all.
Tales has a good plot, but what really makes it are the characters, that are lively, funny, and the voice acting is pretty darn good. If you like Abyss, then I'd try Tales of Legendia -- which isn't as good, but I found it to still be an entertaining tale.
Since you have a DS, I highly recommend Golden Sun I and II, which are quite arguably the best RPGs made for the GBA. I say that because the Final Fantasies were ported on there. And GBA games play on the DS.
I'm an avid RPG fan, and I dunno if you'd like this, but I quite enjoyed Lunar: Silver Star Story complete. XD... It's old school, but quite fun, really.
And lets see... you might try Dragon Quest VIII. It's hard, I kid you not, but it isn't bad. (Not my favorite RPG though.. sorry. XD)
and any Final Fantasy... though the best are probably XII, X, VII, and X-2.
Final Fantasy XII is a little harder than your average FF game, but it's still very, very good. And I loved X, and you can get that pretty cheaply right now..
So to sum it up, I'd highly recommend you get Tales of the Abyss, FF XII, or FF X first.