Talk to him about a good processor, say that you'll need at least 3.0GHz of power on that processor. The GHz (gigahertz) is how much power the processor has. Talk to him about NVidia Graphics cards (preferably newer ones rather than older ones as they're more likely to handle more) as well as the 8 gigs of ram.
Generally tell him you're looking for a good gaming/high performance PC for ____ (insert budget). Then go on to say it'll need about 8 gigs of ram or more, a decent processor and a recognized graphics card.
You can say people have recommended NVidia cards, but just see what he says. Being a computer guy he should know exactly what you're talking about.
He may tell you your budget isn't enough, if he does then just tell him you'll take the best of what you can get on your budget.