What do you currently have that you want to go with new CPU-MoBo-RAM combo?
Do note that $300 is too less for a proper CPU-MoBo-RAM combo for a gaming build.
Though, you can go for these:PCPartPicker part list
/ Price breakdown by merchantCPU: Intel - Pentium G4560 3.5GHz Dual-Core Processor
($82.88 @ OutletPC) Motherboard: MSI - B250M PRO-VDH Micro ATX LGA1151 Motherboard
($63.49 @ SuperBiiz) Memory: Crucial - 16GB (2 x 8GB) DDR4-2133 Memory
($145.99 @ Newegg) Total:
$292.36Prices include shipping, taxes, and discounts when availableGenerated by PCPartPicker 2017-12-06 04:24 EST-0500
This above combo is great for office build though. While you can game on it too but without knowing your old CPU, i can't say if G4560 would be upgrade or downgrade.