304162 Shopping cart

I need to implement a shopping cart system for one of my clients. I have not fixed on a particular cart but I'd like feedback from programmers' experiences. The following features are essential: * It should have slick design that is uncluttered with sharp fonts/layout/images - it is for a computer hardware retailer. * Open Source * Simple and elegant admin interface - must be uncluttered without a thousand features but be able to access the exact functions required. * SEO friendly in all aspects including .html urls * Very high level of security * simple checkout process and constant display of cart contents * must have an article manager so that you can add and remove articles * must be able to integrate with Setcom for payment processing ([login to view URL]) * print PDF invoices Please give me your recommendations for a shopping cart as well as show me what can be done as far as layout and design and the other features mentioned above through real world examples. Please dont just say "it can be done" If you can include Search engine optimisation to get this site into high rankings in google this will be beneficial but not mandatory.