There is no such thing asbuying a good gaming computer.
$1500 is the amount you'd be looking at for a computer of decent caliber. That much will get you a long way in the terms of power these days, much more than even a couple of months ago.
Before you start putting together a parts list, you need to do some research on how computers get put together, what part does each, etc. When you start looking for parts to buy, google reviews and such for them. Don't rush things and put time into research into this, and in the end you'll have something better than your friend's no matter what because YOU made it.