Farcry 2 will benefit significantly yes, that is one of the very few games.
Don't know the Dunia engine.
BFBC2 will benefit slightly, but with i3 paired with only a GTX 275 it won't be more than a couple frames.
Mafia II from benches I've seen usually sees little to no increase going from 2 cores to 4.
Metro 2033 will gain around 5% (likely a single frame) from 4 cores with that little of GPU power.
Dirt 2 gets a somewhat significant increase when using 4 cores, but that goes away when you turn on AA.
My point wasn't that games do not use it efficiently, my point was that an i3 dual core is still enough to keep up with a GTX 275, even in games that are quad core optimized well.
Just because a game benefits from using a quad core doesn't mean it 'need' a quad core to keep up with a GTX 275's framerate, especially when dealing with i-core IPC. Only a few rare games actually 'need' the extra cores w/ a 275.
Too lazy to post benches, but I can if need be.
Lets not forget, I still voted for the i5 quad.