418874 Shopping cart

I am looking to add a professional looking shopping cart to my website. I currently use paypal and authorize.net. I would like a more sophisticated check out that would give the buyer the option in the check out process of using these methods plus a few other popular choices. I want to keep my current website and would need to modify what I have on the website now or create a new site with the shopping cart and link to my current site. I list one of the top five in google search, I don't want to lose my ranking. Any suggestions are welcomed.