For most types of games I think 60 FPS (2 x 30 FPS) should be sufficient. For example cinematic style games like first and third person perspective games. Super Stardust HD is one of the few games where I think 60 FPS can really matter, because with up to 20,000 ultra fast moving colliding objects at once fast screen updates matter much more than having a game where you focuss on just a few slower moving and much bigger enemies at a time.
FPS in other games more relates to being fluent for human perception and Killzone 2 demonstrates this very well at 30 FPS, the enemies are being animated so much more fluid (like the awesome death sequences) than for example a FPS game like Crysis (nomatter the framerate its running). Or for example Tomb Raider: Underworld running on a PC vs the much smoother animated Uncharted 2 on the PS3.
