As others have said, building PCs for a living only works if you specialize in something (SFF is a good one), or only cater to the extreme high end market ($2000+, where HP and Dell charge ridiculous mark-ups for graphics cards and the like).
The other problem is the fact that the market is shifting to laptops. Most consumers don't care about being able to play Crysis with all the eye candy on. Most want to play Bejeweled, Farmville, Plants vs. Zombies, or perhaps Starcraft II, which can be accomplished with your average $500-600 laptop with an entry-level discrete graphics card.
This is something I've wanted to do for a long time as well, but I've held off simply because there isn't enough demand. I would LOVE to build $2000 solid gaming rigs for a living, but unless you're well established, no one has heard of you and won't trust two grand on some company that could disappear tomorrow.