I am wondering what the lowest [standards] spec is for a custom gaming pc

delaro

Judicious
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PCPartPicker part list / Price breakdown by merchant

CPU: Intel Pentium G4560 3.5GHz Dual-Core Processor ($59.63 @ OutletPC)
Motherboard: MSI B250M PRO-VDH Micro ATX LGA1151 Motherboard ($69.49 @ SuperBiiz)
Memory: G.Skill Ripjaws 4 series 8GB (2 x 4GB) DDR4-2400 Memory ($59.92 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($47.69 @ OutletPC)
Video Card: Asus Radeon RX 460 2GB Dual OC Video Card ($89.99 @ B&H)
Case: Thermaltake Versa H21 ATX Mid Tower Case ($36.50 @ Amazon)
Power Supply: SeaSonic S12II 520W 80+ Bronze Certified ATX Power Supply ($48.99 @ NCIX US)
Other: Win 10 Pro 64bit OEM System Builder ($50.00)
Total: $462.21

This is about as low as I would go and be playable @1080p medium or better settings in every game out. This includes the OS "Fully legal version by Microsoft's resell policy" You would only need to use another PC to make a Bootable USB flash drive to install the OS. Sales and rebates that come and go would drop the price $50.
 

delaro

Judicious
Ambassador


Swap out the g4560 for a i5 7500 and upgrade to a RX 470. You could also run a Ryzen 5 1500x for about the same price as the i5. Both option will push you into the 570 range.