As others are saying, at the bargain basement level DIY can not compete with the big names. Now the custom places (whether gaming, silence etc) end up costing more than an equivalent DIY project, though remember they have taken the time to test all components, make sure they work, offer warranty etc. For most of us on this board, I would imagine the time and effort...well, heck, it's like a hobby for us, not work! I like tinkering with my machines (well, until about the 5th time I rewire the case and end up with my original setup
Dell > me
Dell--probably better big numbers (GHZ! GBZ!!! MOOorEE!11!)
Me--quieter, probably longer lasting components, no uneven bottlenecks (like a Dell may have a great CPU, crappy videocard, slow (but lots of) RAM).
Custom shop--winner with warranty and fully installed working OS (not, you know, that I dislike hunting down drivers and making sure everything is happy together).
Me--probably better specs than a custom shop but no warranty or support