It's great to start with low-impact payment processing like PayPal or Google Checkout. That's a low-risk, low-initial-cost way of dipping your toe in the waters of ecommerce. No need to rush in to obtaining a merchant account right away. Seriously!
But at some point if you do things right you're probably going to want to get a real merchant account and start processing your own payments. Up to a point, it's actually less expensive to use third-party processors like PayPal, but after awhile if the business grows it becomes much less expensive to run your own merchant account. The difference can be staggering, and only gets larger the larger the business grows.
So while it might be tempting to start with something non-ecommerce (like Drupal or Joomla or whatever), honestly your best bet would be to plan for the future, take the plunge, and set up real store software. Most will integrate nicely with PayPal and/or Google Checkout, but they will also give you the option of fairly seamlessly migrating to a merchant account on down the line (or adding on -- nothing says you have to stop
accepting PayPal just because you also have a merchant account).
Otherwise, when/if you get to that point, you'll find yourself having to recode and reconfigure your entire store in order to migrate to a real ecommerce platform, or going through significant gyrations to try to shoehorn your existing CMS into becoming an ecommerce platform, which it was likely never designed to be.
Store software is no harder to setup or configure than Joomla, in my experience. In fact, if what you're setting up is a store, using "real" store software will probably be easier and more intuitive than trying to juggle Joomla or Drupal into doing something they were never really intended to do.
Zencart, OScommerce, the alternatives mentioned previously in this thread, and many others... all are good choices. It's just a matter of checking them out and determining which one(s) can provide the options you need to accommodate the types of products you plan to sell.
But whatever you decide, I strongly recommend you use a product designed for ecommerce, not a general-purpose CMS.