[BradleyT]

ZenCart is just OSCommerce with some of the "nice" contributions already installed. OSCommerce is pretty easy to install and upgrade provided you're comfortable modifying PHP files. I'd check out templatemonster to find a nice looking template if you choose OSC or ZenCart.

Overall I'm pretty happy with OSC (actually I'm using CRELoaded Pro which is a $$ version of OSC) so far but I'm still in the process of getting products in my site and getting the template I bought to be HTML 4.01 valid.