Playstation 3 - /v/'s Recommended Games Wiki
Out of the PS3 games that I have played, I recommend Valkyria Chronicles and Demon's Souls for your RPG criteria. The story for Valkyria Chronicles was kinda blah to me, but the gameplay is really fun.
I haven't played Nier or Resonance of Fate, but I heard both of them are pretty good.
For action, I guess try to get Uncharted 2 and Vanquish at a low price. They're really fun, but there's not much content imo.
If you like action with stealth, you can try Assassin's Creed 1 and 2.
Assasin's Creed 1 is a bit boring but still somewhat fun. The 2nd game is better, imo. If you like backstabbing people and jumping around like a monkey, then you should have some fun with them.
As for Metal Gear Solid 4, try it out if you had beaten MGS 1, 2 and 3. Personally, I think it's the worst one in the series due to LOLMOVIE, but I shed some manly tears while watching some cutscenes.
I've only played a little of Red Dead Redemption and Dead Rising 2, but they were pretty cool from what I've had played. So you can try them out if you want.