I bought my 800 t-bird and KT7A-RAID from www.mwave.com The have good prices and their customer service was top notch.. I had problems with my credit card and they helped me over a few days that it took to fix the problem. They even shipped mystuff next day overnight becaus of the problems Iwas having.. The problems were on my end. I got e-mails after they recived my order, then again when they processed it and then when they shipped it. I will deffinatly shop there again. I heard www.azzo.com is very good also. You will see more posts in the Vendor Discussion area.
Another good site is www.comp-U-Plus.com nice selection and reasonable prices and they don't kill you on Shipping like some places..
www.compuplus.com is a great site, but the prices are higher for CPUs than they should be. I got a mobo at www.microbarn.com , they have a smaller selection, but they are great mobos at great prices. CPUs seems to be cheap at www.upgradeplanet.cpm. Also, anandtech does a weekly cpu, memory, video, and motherboard price guid in which they have the cheapest sites that they have not had bad reports from their users.
I like Multiwave Direct and Newegg the best. i've dealt with Multiwave several times over the years and recently had two good experiences with Newegg. Try Newegg first because they have a lot of free shipping deals. Here's the links:
First places I check are always the local computer stores. I like to get my stuff right away, and if the price is within a dollar or two of what it would cost buying it online (product cost+shipping) I'll buy it there. I also have a tendency to wait and buy stuff at those Super Computer Sales if there's one coming up. Otherwise, if I have a lot to gain by buying online I will just use Pricewatch in tandem with resellerratings.com. I don't buy from vendors with a less than 3.25 out of 7 rating. Some sellers I have dealt with successfully are: Neutron, Beyond Computer, and USSA.com.
Unsuccessfully (as in I wouldn't do business with them again): Memory To Go, shophereusa.com