Congrats on your first build! I think I was also about 14 when I built my first. It also just had an AMD APU until I scrounged up enough money for a GPU.
Awesome budget build. That i5-3570 definitely has some life left in it. If he ever wants an upgrade down the road, something like a 1050ti would work perfectly in that system.
If I were you, I would get a slightly different, cheaper CPU and get a slightly better GPU. But, with the build you have right now, you can upgrade just the GPU later.
The case and PSU are good, so don't worry about those.
I brought you down to an i5-8600 instead of the i7 so there'd be enough room in your budget for a 1060 6GB. If you're just gaming, you're not going to need the i7.
There could be a problem with that second RAM slot, yes.
If either RAM stick works in the 4th slot, but neither work in the 2nd alone or together (with the other in the 4th slot), then it's likely a problem with the motherboard.
Hopefully, that makes sense. It's just process of elimination. if both of the RAM sticks work by themselves, then it's likely the motherboard.
The CPU I'd recommend all depends what socket the motherboard you have right now is. You can get by today with 4GB of ram and two cores just fine for basic internet browsing.
I'd definetely look into getting an SSD over a HDD. You won't regret it. Just like your RAM, get a 240GB SSD to get yourself started, and add the 1TB in later.
Does he have an SSD in his old build? Adding an SSD would make the most noticable jump in performace in normal use over any other part. The i5-2500K is still a great CPU.
I'd say, with a budget in mind, just throwing in an SSD and upgrading the GPU would be more than enough to breathe some new life into the system.
