Here what I've tested so far and I could suggest :
Adventure :
3D Dot (2Go) (only works on external HDD)
Castlevania (11Go, worth it) (my best PS3 game so far
)
Racing :
Wipe out HD + Fury (Europe had the Bluray edition instead of PSN download) (2Go)
RPG :
Disgaea 3 (2Go)
Atelier Rorona (if you like Atelier series, this one is more item collecting/drama/RPG Text than adventure) (3Go)
Other :
Katamari Damacy (9Go) (I still prefer the very first one on PS1)
Fighting:
Naruto ultimate ninja storm 1(full naruto story) (4Go)
Naruto ultimate ninja storm 2(full shippuu story) (8Go)
games I would like to test:
Folklore
Uncharted
Valkyria
Star Ocean 4
maybe LBP and FF13, but they don't interest me.
PS :
Do not suggest GT5, it needs Firmware 3.50 and he obviously want to jailbreak
PPS :
- Before downloading a game, verify it's compatibility here :
http://www.psjcl.com/
- The size is the rar'ed download size (you will have to extract, and often install more data to internal, like naruto 2 need 8Go of game + 4.5Go of installed data). I hope you will buy a 320Go slim
If you can, ask friends to lend their games so you can dump them yourself instead of downloading.