http://pcpartpicker.com/This site is awesome.
It lets you pick parts for your own build, providing a list of all of the types of parts that includes all of the necessary things you need to know about it, such as the specifications, price, rating, and price per gigabyte in the case of hard drives. Everything can be sorted in ascending/descending order, whether you want it to be by rating, price, or name. If you want more specific information about a part, including detailed specifications, pricings from different merchants, and even graphs of the part's price history from many different merchants.

GUYS
THIS IS MY FAVORITE PART GUYS
GUYS
HOLD THE PH
GUYS
SERIOUSLY
HOLD THE PHONE
HOLD THE f**kING
GUYS
PHONE
HOLD THE f**kING PHONE
ARE YOU LISTENING?
It will not show parts that are incompatible with your current selections. Takes out a nick in the shoulder, eh?
Try it out.