Of course even if you do all the things you said to a pc, MOBO, CPU etc, space, fans, you're still probably going to experience some LAG??
Not if you've purchased the latest and greatest for an existing game. But since technology advances so quickly, new software may not perform as well on older (one week older than itself) hardware. For hard core gamers or programmers, its just a way of life and you learn to accept it. In fact, you learn to welcome the new technology because it removes some of the old bottlenecks.
So uhm, where is the good part for us gamers in high end performance PC's?
Most folks don't compete on that level. Get a machine that plays the game you like well. Maintain that hardware and it will serve you well for years.
Don't forget that lag is also different in the multiplayer realm. Other factors are introduced into the circle when you hook up to a foreign server. Factors that are not induced by your machine but by the server quality and it's proximity to you.