G4560 is similar to the 2500K at stock clocks and the I3-6100. The 2600K adds 100 MHz, 2MB cache and hyperthreading to the 2500K.
http://wccftech.com/intel-pentium-g4560-ultimate-budget-cpu-65-usd/
8GB is the winner, though 16GB is nice to have.
https://www.techspot.com/article/1043-8gb-vs-16gb-ram/
There is a possibility that the b250 will be able to use the new 1151 6 core 12 thread coffeelake cpu, though not confirmed. An I7-7700 4 core 8 thread cpu will be a lot easier to find in a year or two when you need more power. You'd have to overclock the 2600K with a decent cooler and if something happens to the motherboard you have no warranty. If more power is needed you have to update again.
If this is a gaming PC go with the g4560, b250 with 4 ram slots, and ddr4 2400MHz. No overclocking on the new board. Save $30 every other week for a year or two, drop in a locked I7 and next generation nvidia 11 or 12 series and double the ram.
https://www.newegg.com/Product/Product.aspx?item=N82E16813130989 $11 more than the 2 slot board, adds USB-C, better pci express layout you wont lose an x1 when GPU is installed.