If you don't have any really unique or specific needs that might require custom features, and you don't have like 5,000 products to sell, then I might suggest getting your feet wet with BigCommerce. I've done several stores using things like xcart, magento and cscart, and other things like pinnacle cart and bigcommerce, and when it came down to the fastest, easiest, and fewest hassles to just get a shop up and running, bigcommerce took the cake.

Be prepared though there will be features you don't like and can't change. And you will more than likely grow out of it. But the bottom line is that you won't know what you want to change until you've had some experience with it.

The only widely used cart I haven't had any experience with is Foxycart, so maybe someone can offer some opinions there.