And the answer is...(drumroll please)...
NEWEGG!
(...gee, what a surprise...)
Seriously. They stock everything (parts wise-they don't carry many printers or digital cameras or any complete systems, but they have 200 different video cards or so, and equivalent amount of choices for motherboards, processors, sound cards, monitors, cases, etc.), have a great website with pictures of the products, reviews, and detailed descriptions, so you know exactly what you are going to get, ship within a day of you placing your order, take back returns with no problem, are usually within 5% of the lowest Price Watch price from some scumbag, don't overcharge on shipping, pack things correctly, and have a 9.48 (out of 10) on Reseller Ratings, with 1400+ ratings. You CANNOT get better than this.