anyone who says dxc sucks, probably never played anything in the series before symphony of the night. if you're the same, but you're at least open to a challenge(and really it is one of the easier, if not the easiest, of the oldschool platformer types), then the rondo of blood remake won't be so bad for you. if you have played and enjoyed castlevania 3 and bloodlines, you should love rondo. especially since the remake has all sorts of stuff you can look for throughout.
crisis core - if you're a fan of final fantasy, don't go into this expecting more of the same. in fact, i think it generally disappoints anyone who enjoys final fantasy's usual gameplay.
god of war - pretty much the same as anything else in the series you've played before, just apparently shorter. not everyone likes it, but you might look into ghost rider as well. it's said to play the same way.
other reccomendations:
non exclusives:
puzzle quest - addicting as hell, if you enjoy rpgs and bejeweled type puzzles
valkyrie profile: lenneth - same as the psx game, just with 3d rendered fmvs.
x-men legends 2/marvel ultimate alliance - same as the console counterparts, only i think x-men is missing online play, but both games have exclusive characters
marvel trading card game - guess you'd have to enjoy card games in general, but it's still a good strategy game that tends to involve cards of obscure favorite characters.
warriors orochi - like the console versions, just graphically inferior, and has comic style cut scenes. avoid the other koei warrior series', unless you don't mind a more board game/strategy-ish style.
sid meier's pirates - basically gta on the open seas
megaman power up & megaman maverick hunter x - 2.5d remakes of the first games in the megaman series'(counting x as a different series)
ultimate ghosts n goblins - 2.5d game in this series
exclusives:
brave story - excellent rpg, and this is coming from someone who generally hates the random/turn based combination, which it has
jeanne d'arc & wild arms xf - normally i don't care for srpgs, but these 2 are pretty good
exit - puzzle game, based on getting people out of dangerous situations
monster hunter freedom/2 - not my type of thing, but people are really into it, so i dunno
guilty gear judgment - has one of the fighting games, and a beat em up
sonic rivals/2 -racing, but despite the racing aspect, feels like an oldschool sonic game
metal gear ac!d/2 -card stealth games, works out oddly enough
metal gear portable ops - normal metal gear stealth game
crush - puzzle game that allows you to switch between a 3d view and 2d one on the fly
dead head fred - gory platforming/brawling involving switching heads for various situations as needed
it really is an excellent handheld. as for the memory stick, whatever size you get, if you fill it, remember you can always use a pc as a backup hard drive.