>>> With a modded PSP you can play almost all the good PS1 RPG games..
Chrono Cross, FF7, FF8, FF9, Xenogears, Suikoden 1 and 2, etc.
As for PSP titles you have Disgaea 1 and 2, Valkyrie Profile Lenneth (these are my favs), and other games that seem to be popular like The Legend of Heroes: Trails in the Sky, Valkyria Chronicles, Phantasy Star, .Hack games, etc..