Implementation depends largely on the payment model you're using. If you're charging up front for the downloads, you'll need to use a shopping cart + AIM (Advanced Integration Method). If you just keep track of downloads and charge whenever the balance goes over a certain point (to reduce credit card fees), you'll need to use CIM (Customer Information Manager) to keep track of the billing info so you can charge their credit card as necessary without storing it locally (which is dangerous and illegal) or having them re-enter it every time (annoying). Either way, it may be difficult to find a prepackaged solution that does exactly what you need, though I suppose you can download all the solutions that say they support digital downloads and implement them one at a time until you see something you like. Try the link Michelle gave you and search for the word "download" on the page.
