Well theres a TON of rpg's on the PSP xD Some are ports, some are original, some are multi :P I'm massive on RPG's so thats a delicious area for me xD
Jean D' Arc! I've heard nothing but great things from this game :P I need to pick it up, I Just have so much cued up at the moment xD
I forgot which of the Tales was on the PSP, but my friend is thoroughly addicted to it,
I picked up Valkyrie Profile a while back ago, thats definitely another good one :)
Sometime this year, keep an eye out for Phantasy Star and Monster Hunter, both action RPG's with Multi-player :)
Star Ocean 1 and 2 are also due out this year, when however I'm not sure.
The Remakes for Final Fantasy 1 and 2 are there as well, I didn't remember them much on the NES and SNES so I picked those up to re-touch.
If you do dig the Strategy RPG scene (which from your taste in tactics I might assume not :x ) Disgaea is TOTALLY up your alley :)
Then of course, how could I forget about Untold Legends xD Action RPG's with Multi-player co-op I believe :) Those games are damn awesome sometimes ^^