The difference between 4 and 8 cores, yes, the 8120 is an 8, is the more cores you have, the more applications you can handle. Or in gaming, 8 cores will improve performance over any other less cores. However when it comes to AMD vs. Intel, Intel always comes out on top.
Actually, there are only a few titles out there that can take advantage of 8 cores. The 4.2ghz FX4350 would be better in gaming 85-90% of the time over the 3.1ghz FX8120.
Didn't AMD announce that there would not be any new Steamroller FX cpus, only a Piledriver refresh? I wish they would change their mind. The Kaveri apus are pretty good, they're just overpriced at the moment.