apples and oranges since both CPUs companies design their products differently. Really want you want to look at is performance for your dollar. Your best would be to start with tom's best CPU for the price but for under $200, you can get a solid intel i5 that will offer better gaming performance then the FX-8
First of all you seem to be hinting at the thought that more cores and higher clock speeds are better but that is just not true. More cores isn't always better it really depends on what you are doing. If all you are doing is playing games you don't need a six or 8 core processor. Next clocl speeds don't matter so much either. The fact is that Intels microarchitecture is faster and more efficient than AMD's allowing it to do more work faster despite being clocked slower. What this means is the 3.4 GHz I5 will beat out the 4.0 Ghz Piledriver in games.
Next there is no point in getting the I7. There is very little difference between I7 and the I5 except that the I7 has more cache and hyperthreading two thing games don't make use of. There is no need to spend an extra 100 dollars on the I7 over the I5 because you are paying for features you won't make use of.
Now as for which to get I would go with the I5. You look at the benchmarks and you can see the I5 out performs the Piledriver in games.