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off (as Dell and friends tend to overcharge for outdated or underpowered components).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.
Then you'd be mad to go with the NewEgg solution. Dell's offering would include a single warranty, be properly developed to have good airflow (always hit and miss, more often miss, with home builds) and is pretty much guaranteed to "just work". Home build
machines need to work out a lot cheaper to be worthwhile, require a lot of expertise to get right and even then are subject to a fair bit of luck.
And I'd disagree about high end hardware, the more powerful the machine you need, the more important it becomes to get a system with well designed airflow and that's tricky with a case you buy by picking what looks nice in a webpage. And big OEMs put a lot
more effort into checking alternatives, such as the overall performance characteristics of hard drives etc, which the specs on NewEgg aren't really going to show you.