If someone's doing a "home built" it means they're after components that are more suited to their requirements; there really don't exist many bottlenecks and in my experience OEMs deliver worse performance/currency than DIY boxes because they build for the
80% case which doesn't need "overall performance": they're engineered to reduce support costs and keep manufacturing costs down. I may remind you that many OEMs were guilty of still using Intels not-good-enough-for-Vista IGP chipsets well into 2008 whereas
everyone who builds their own were using cheap 'n' cheerful (yet in comparison, blazingly fast) entry-level GeForce cards.
Right now I'm using a box I built by myself, Q9450 with a GTX280 that cost me significantly less than a lesser machine had I bought it from any of the major OEMs (or even Alienware).
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.