photonboy :
Budget.
Agreed, we totally need a budget for the system or it's impossible to give the optimal balance of parts. For example, you don't get an $80 CPU and a $400 graphics card.
I have been recommending the X4-760K at times but keep in mind it's the CPU only so you'd want a half decent card perhas the GTX750Ti.
Other:
APU's also share System RAM. The GPU in the APU not only uses some of this memory but it also needs more expensive memory (i.e. 2133/2400MHz) or the GPU can be bottlenecked. Thus an APU's value might not be as good as it first seems.
There isn't a single APU I can recommend for gaming.
Good overview.
However, keep in mind that if this guy is considering an A6-6400K at all
his budget is likely very low. On a very low budget, an APU can be a great option, as it can get done basic graphics, even on cheap 1600mhz RAM for old games and online games and stuff.
On a low, low budget, the A6-6400K is a pretty good option. But the Athlon x4 740 is a better deal for the same price if he already has a graphics card or he is planning on buying one anyway.
Neither are great at gaming at all, but at a $70 budget
without a GPU, you can't beat the A6-6400K. But for $70
with a GPU, get the Athlon x4.