Ok, good, you already got Castlevania and Dissidia. So you're off to a good start. I gotta say listen to everyone here. The PSP may have slim pickins, but the good ones are all stellar.
Patapon is my first recommendation. Its a 2D RPG/RTS Rhythm game. Don't give me an excuse like 'i don't have good timing/rhythm.' I didn't think so either, but you catch on right after level one. It gets frustrating after you beat the final boss the second or third time, but who cares? It was a fun time.
Im sure you've heard of Puzzle quest. Its best played Portable but both PSP and DS versions suffer horrible slowdown. But it really hasn't stopped me, because you don't really need good framerates for swapping gems. Go buy this, my Number 2 recommendation.
Metal Slug anthology is a great Bundle. Almost as good as the orange box. Its got all 7 Arcade Metal Slugs. Loads of fun, and even with 30 credits you'll be hard pressed to actually beat any of the games, unless you're some kind of master.
Like Gollgagh, i highly recommend Lumines II. The best version is on the PSN, but this is best portable, and best with headphones. *as is patapon. Definitely use headphones. Eaiser to get in the beat.
Quality RPGs are kinda a rarity on the PSP. If you're into Final Fantasy Tactics, definitely go for that, but i wasn't a fan. Jeanne D'arc was another Strategy RPG, but i have yet to find one to emulate Shining Force (that's my favorite). Valkyria Chronicles 2 is also coming in the summer. And the PSN is overfilled with Classics (most crap), but you can find some good RPGs like FFVII, FFVIII, Suikoden, Wild Arms 1 & 2, and all the original spyros and crash bandicoots.
I think that's a good enough bunch.