for the both of card HD4670 is a clear winner (should be on par with GeForce 8800GS @ 9600GSO). as for the game, the game like crysis are GPU hungry game. even HD4870 X2 / GTX295 are struggling in this game for max setting.
8800GS and 9600GSO pisses all over HD4670 dude. ATI is weak. Thats a fact. They got good clock speeds and fake GDDR 5 but still ATIs new 5 series are getting raped by the last gen nvidia 200 series ^^ But when it comes to 8400GS VS HD4670 the 4670 winds clearly. I had a pc with the 8400GS before and the pc i had before my current used a HD 4650 which was atleast 5 or 6 times more powerfull then my old 8400GS. So yeah HD 4670 is far more powerfull.