www.tomshardware.com
Tom's Hardware and anandtech are by far the most informative computer hardware guides that can be found on the internet. This site sends out a biweekly email that really can't be beat... if you're into that kind of thing I mean.

www.anandtech.com
AnAndTech is another good computer hardware online magazine. The forums here are awesome. Check them out if you need answers from some knowledgeable nerds, or if want to find the best way to rip off your local Staples.

www.newegg.com
Hands down, NewEgg has the widest selection, cheapest prices, and fastest shipping of any other online computer hardware store.