Is it a good Gaming PC ????

PCPartPicker part list: https://pcpartpicker.com/list/6PsLFd
Price breakdown by merchant: https://pcpartpicker.com/list/6PsLFd/by_merchant/

CPU: Intel Core i5-6500 3.2GHz Quad-Core Processor ($190.99 @ SuperBiiz)
Motherboard: Gigabyte GA-H170-GAMING 3 ATX LGA1151 Motherboard ($80.98 @ Newegg)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-2400 Memory ($74.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($48.89 @ OutletPC)
Video Card: XFX Radeon RX 480 8GB GTR Video Card ($249.99 @ Newegg)
Case: Deepcool TESSERACT BF ATX Mid Tower Case ($34.99 @ SuperBiiz)
Power Supply: Corsair CXM 550W 80+ Bronze Certified Semi-Modular ATX Power Supply ($54.51 @ Amazon)
Total: $735.34
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-11-29 15:07 EST-0500

Slightly over $700 unfortunately... I tried to sneak in an i5 6500, slightly over budget. Could easily swap this out for an i3 6100 to get well under budget and get an SSD + slightly nicer case.

I will point out. With this being a custom build, it does not: Include the OS, include a monitor, include a keyboard, or include a mouse. If those are things that are also needed, let us know.

That build is not bad, I'm not a fan of the x4 CPU in it, but it is not bad.

Yes, that's easily upgradeable... but

PCPartPicker part list: https://pcpartpicker.com/list/jLcbTH
Price breakdown by merchant: https://pcpartpicker.com/list/jLcbTH/by_merchant/

CPU: Intel Core i7-6700 3.4GHz Quad-Core Processor ($294.99 @ SuperBiiz)
Motherboard: Asus H110M-A/M.2 Micro ATX LGA1151 Motherboard ($40.98 @ Newegg)
Memory: Team Elite Plus 16GB (2 x 8GB) DDR4-2133 Memory ($69.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($34.99 @ Amazon)
Video Card: MSI Radeon RX 480 4GB GAMING X Video Card ($179.99 @ Newegg)
Case: Rosewill Galaxy-03 ATX Mid Tower Case ($19.99 @ Newegg)
Power Supply: Thermaltake SMART 650W 80+ Bronze Certified ATX Power Supply ($34.99 @ Newegg)
Total: $675.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-11-29 15:40 EST-0500

If I skimp on parts too, I can easily make that under $700. Again: No OS, no keyboard, no mouse. But if you skimp on parts that make the rest of your hardware not live up to their standard; yes you can easily do it. I'm not a fan of the (likely) cheap PSU they will put into that case. Not a fan of the fact that it's an H110 motherboard in it, and not a fan that it's a 4GB variant of the card when an 8GB card could easily be put in. That being said, that machine is much better than the first machine you were looking at with the upgrade to the i7, and the VRam downgrade will likely not be an issue for a long while unless your son wants to play on a 4k screen.

Not many on this site are fans of pre-built machines because they put in good buzzword hardware and then skimp out on the rest of it to make it fit a budget... and then sell it overpriced. That machine is normally $1100, when I could've made it up to their spec for under $700. I could easily make it much better than theirs for the same price. Kinda awkward...

Again; that said, that machine is much better than the one you were looking at, and if it's simply something to plug and play, it will be more than sufficient. Do you know what games your son enjoys?

That machine will definitely be more than enough for his purposes, then.

If it's without the peripherals:
PCPartPicker part list: https://pcpartpicker.com/list/PsW4Yr
Price breakdown by merchant: https://pcpartpicker.com/list/PsW4Yr/by_merchant/

CPU: Intel Core i5-6600K 3.5GHz Quad-Core Processor ($219.99 @ Newegg)
CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.99 @ Newegg Marketplace)
Motherboard: Gigabyte GA-Z170X-UD3 ATX LGA1151 Motherboard ($115.98 @ Newegg)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-2400 Memory ($74.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($48.89 @ OutletPC)
Video Card: XFX Radeon RX 480 8GB GTR Video Card ($249.99 @ Newegg)
Case: NZXT S340 (White) ATX Mid Tower Case ($66.99 @ SuperBiiz)
Power Supply: EVGA SuperNOVA G2 650W 80+ Gold Certified Fully-Modular ATX Power Supply ($75.98 @ Newegg)
Total: $887.80
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-11-29 16:41 EST-0500

White build. Could swap for a different color. I am 95% certain that the GTR comes with two white LED fans preinstalled on the shroud.