i7 is a quad core CPU that uses "hyperthreading" to do a pretty good impression of 8 cores.
i5 is a quad core without the hyperthreading feature.
i3 is a dual core with hyperthreading that emulates a quad core.
FX4100 has two pairs of "modules" that share resources.
Generally speaking, Intel's "i" series CPU's outperform AMD's FX CPU's. (Although AMD may have found a price-point-gap in Intel's game recently with their new "Pile Driver" FX chips. -like the FX6300 )