Pc Part Picker

This may be the only website you need to help pick which parts you may need to build a computer. It allows you pick which parts you want and check if they are compatiable with all your other parts. If it is not compatiable with your computer it will tell you and might give you suggestions prior to picking the part. It also gives you sites the part is avaible to purchase. It is an extremely convientent website.

Choose my PC

Same concept as the link above but it generates a computer build for you with certain conditions set. Such as money and future plans with the computer. The build suggested are for as far as I can tell for gamers. The first link is better if you just want basic parts.