Reed wants a gaming computer and he wants to know if he should buy one or build one. Leo says that it used to save a lot of money to build one. That's not true as much now, as there's no overall warranty or tech support that can help if the computer doesn't work. Reed would be his own tech support. And with various part manufacturers, nobody takes responsibility for issues.
On the other hand, it's a great project for learning how a computer works. Leo recommends going to PC Perspectives and looking at the HW Leaderboard. The Hardware Leaderboard lists parts in four different part categories that have been vetted to work together. So he can put together the system within his budget and know the parts work together. And if Reed wants to record Let's Play videos, then upping the power of the processor would be a good idea.
Leo recommends looking at buying a computer, and not a "gaming" computer because he'd end up paying a premium just because it's for gaming. A fast SSD is also important.