Many games uses max 2 cores than the difference is made by the most powerfull core.
A dual core processor destroy a single core processor, even if the dual core have worst ipc (instruction per clock. Unit of measurement for cpu power)
Quad core vs Six core is different, few games support many cores then the most powerfull cpu (ipc wise) will beat the other one with SAME Clock speed
In this case i3 3240 vs fx6300 you have to look at many factor
- price i3 about 125$ fx6300 about 120$
- i3 3,4Ghz clock locked with no boost, fx6300 3,5Ghz with boost and fully unlocked
- some games start to manage 4 or plus cores. Bf3 for example, or crysis 2 and the future is going to multicore games (console have 8 core processors)
In my opinion, now is better FX6300 than i3 3240
price, performace and future wise