This article link was in the Friday Shortbread. Sadly they don't include any modern CPUs but it does show that some modern games definitely benefit from quad cores. It would have been nice if they'd included something like a 2c/4t i3 and a quad core i5 for comparison sake, but if you really want to do the work you can extrapolate from various reviews. It also shows that with a modern card, 'old' CPUs still give playable framerates - the Core 2 architecture really was a huge leap forward and is still usable.
Anyway, yes, older video cards that at least have unified shaders can still be decent. Running out of VRAM might be the biggest issue and cause the greatest performance drop-off. Plus if you only play older games then an older card won't suddenly become slower, power draw and noise then become the main upgrade reasons.