Expanding upon what Ph?bus and the others said here: I purchased a pre-built from CyberPower at Christmas '09 -- and let me warn you now, if you DO order from them, always put aside the extra $50 or so that it takes to expedite the shipping down to 3-5 days or you will be waiting up to 2 months for it to be built and shipped if it's a busy holiday/season.
That said, they're not a BAD company. It's just that you pay premium prices for backwater parts. I've actually opened up the rig I ordered and unless you specifically pay extra for the brand you want (which isn't always an option; sometimes that brand isn't an option to select, as they have a very small selection of top-notch brands), you'll get low-quality components from brands you've probably never heard of, which may not be a gamble you want to take with the price you'll be paying.
I figured the math out and -- typically, it is about 20-55% more PER component on TOP of the shipping charges, assembly fees, and any applicable taxes. Well over a hundred bucks for a 700W Corsair PSU? $400 for a GTX 560? wat
Honestly, your best option if you don't want to build the entire thing yourself is to order a barebone kit that suits your budget, then put in your own quality components. The rig that I ordered hasn't fallen apart yet, but the mediocre quality of each part and "who the hell are they?" names of the brands on the components is a little concerning.