Prices vary upon selected attributes

I am setting up a shop to sell downloadable products... PDFs to be precise. Each product will be available in both high res and low res versions each with their own price and the two prices will vary from product to product.

I'd like the customer to be able simply to view the product description page, select which resolution they'd like to buy, have the price update automatically, and checkout as normal.

What would be the best way to configure the site in order to achieve this? "Characteristics" doesn't appear offer a price variation (unless I'm missing a trick here) and "Options" does but only by adding additional products.

Thanks very much

You, sir, are a star! I knew it could be done somehow, and I looked in that part of admin but for some reason it didn't really click with me... thanks!