AMD has managed to get more physical cores and faster speeds in some cases, but Intel is more efficent at doing stuff in each of those GHZ, so the current winner is Intel because an Intel 3.5GHZ vs an AMD 3.5GHZ chip, the Intel is going to do better in most benchmarks and especially gaming.
Now if you're doing rendering, or stuff that can use all the cores, then an 8-core AMD starts to become attractive because then you have 8 cores at 3.5ghz, and even though the Intel can do more per core, the 4cores of the Intel vs the 8cores of the AMD, the AMD will.
Gaming though, most games only use 2 cores and starting to use 4, so in a I5 vs FX battle, the I5 will be faster, it can do more per core. In multi-core apps, the FX will do better overall.