[instinctual]

well first you're going to have to combine the products.tpl and product.tpl so that you can choose options on the products list page.

Then, when you click "buy now", you will be sitting on your "cart" page to review your order - from there I would just integrate the "register.tpl" form into the cart.tpl template, and they can choose to continue shopping or fill out their billing info (use anonymous checkout so no pass needed) and you could turn on "save cc info in profile" so they could put in CC right there too.

Then, you could alter cart.php to skip the "summary" step and pass along the CC info as entered as hidden variables, and go straight to the CC payment gateway.

Not doable unless you understand php and smarty, doable if you do. The x-cart team is always a good place to start, or posting in the professional services requests forum.