Some RPG? You're probably thinking of Demon Souls! Or perhaps Valkyria Chronicles. Both well worth considering. Also if you like your jRPGs, Tales Of Vesperia sounds good and I'm keeping an excited ear to the ground about FFversusXIII.
Theres plenty of good exclusives to keep you entertained and more on the way, which you normally wouldn't be able to get your hands on from having a PC/360 setup. I'd keep an eye out for a bargain or just hang fire for a bit to see if there is a price drop at TGS.
If I was going to go down the path of worthy stuff you can't get your hands on for the moment on your current setup it'd be:
Valkyria Chronicles - best thing to come out of Japan in a long long time.
Demon Souls - not gone hands on with this myself but its proved to a bit of a storming cult success. Been reviewed well too.
Motorstorm 1 / Pacific Rift - just pure arcade off road racing fun with no faff like weapons or tricks. Perfect. Looks stunning too.
Killzone 2 - Single player can't match up to something like Bioshock (my current benchmark) but the multiplayer has been the best FPS multiplayer I've touched on a console.
Flower - nice bizarre indie distraction
Wipeout HD - its Wipeout! Gorgeous game and nice price too.
Siren:Blood Curse - Survival horror action like it used to be done, nice if you're jaded by Resi 5
Warhawk - now I love this personally. Its not a technical showcase but its just pure online fun. Well balanced weapons and maps with a great sense of scale. Troops, tanks and air... its all covered.
MAG - this is coming soon, if Warhawk felt like a battlefield, I guess this will feel like a proper war! 256 online FPS battler with proper command structure and objective play. Keeping an eye on reviews for this.
Infamous - next on my shopping list. Plays like a dream, intuitive free running action, satisfying to pull off, pulls off a world not too dissimilar from the detail levels of GTA4. If you fancy Uncharted, then check this out.
Uncharted 1 & 2 - 1 certainly did the Hollywood adventure flick thing well. Good script, good acting, tight gameplay.
Folklore - Got a bit of cult status as far as being a bit different for an action jrpg. I've got this sitting about in my list of to play stuff.
White Knight Chronicles - Seems like its been pretty well received. FFXII-ish from the screenies I've seen with offline and online play. Waiting for the EU release.
LittleBigPlanet - dead cheap now, full of character
I mean thats off the top of my head of things I would consider. Other people might recommend stuff like Resistance 1 & 2 but I've not really played them and I've probably left some huge gaps in there, especially with the PSN content (things like Fat Princess, Pixel Junk games, SuperStardust).