When I originally launched the Buyer's Guide years ago, the major distribution channels for PCs were business-oriented storefront dealerships and mail order. The dealerships had (and still have) high overheads and higher prices. Accordingly, I recommended mail order.
I still like mail order, especially for techies on a tight budget. Publications like "Computer Shopper" (and their web site at http://www.netbuyer.com) are a great way to get a feel for prices, and these days most mail-order outfits with enough cash to advertise on glossy paper are good risks. The online version of my Buyer's Guide has details on how to protect yourself when buying mail-order.
These days, though, I'm also a fan of computer superstores -- outfits like CompUSA and Circuit City that sell hundreds of machines a day out of warehouse-sized premises packed to the ceiling with discounted hardware. These obviously have more overhead than mail-order outfits, but their price premium over mail-order is small. They make back a lot of their margin on computer games and small accessories like mouse pads, cables, and floppy disks.
So if you shop carefully and don't fall for one of their name-brand "prestige" systems, you can get prices comparable to mail order with the comfort of knowing there's a trouble desk you can drive back to in a pinch. (Also, you can see your monitor before you buy!)