I don't know about this... Because of simple volume, OEMs just have more buying power than any individual. Dollar-for-dollar, you simply cannot beat the hardware cost of an OEM. I recently bought a Dell desktop for $350... and there was just no way I could
spend that cash for that hardware...
And OEMs just can put together better components than me... I can guess if a certain motherboard would play well with a certain video card... If I'm wrong, I'm out a 1/3 of the cost of the machine.
You might be interested to know that I was just able to spec out a machine practically identical to Dell's cheapest desktop ($249) on Newegg for almost exactly $249. And once you start talking about higher-end components, building yourself starts to pay
off (as Dell and friends tend to overcharge for outdated or underpowered components). It's hard to find a system without integrated graphics from Dell now, even.