A8-6600K and 760K are very similar as they are both Richland-based. the A8 is an APU which means it has an IGP. The 760K has no IGP. I believe the A8 is 100MHz faster but it's also ~$20 more expensive. Between the A8 and 760K I would choose the 760K.
FX 6300 is a 6-Core (3 Module, 6 Thread) AMD CPU. It is only ~$99-100 and one of the best CPU for the budget. Due to having more cores the FX 6300 will do better when gaming and streaming.
Pentium G3258 is only ~$70 and it's a Intel Dual-Core with no Hyper Threading. It is a great CPU for gaming though for the money. With the G3258 you can upgrade to an i5 later allowing the budget PC to become a high end PC after a CPU upgrade.
In my opinion my choice would be the FX 6300 vs...