Dynamic Drop Down Boxes

Hi there, Basically all i require is a simple drop down box cart website that gives me options and chnages prices of the laptops that i sell. At the moment iam not sure how many options i need but it should be? customizable so i can edit it. so for the first drop down box i will need: Sreen size: 10inch, 11inch, 12inch 14 inch then for the next dropdown box it should be related to the box above, so basically if i select 10ich the next box should have different options than the 12inch and the prices are shown. etc Furthermore. it should? calculate? the price of each item and give a final total price at the end of the script? which? is then payable through Paypal or other options. As well as this it should all be in a website on the main page with a few other links at the top to contact us and otehr information, etc. this should be done very quick and very cheap.