Go with osCommerce. Agora and its sisters are not really worth it, by the time you get it set to go you could have osCommerce working as well, and have 100 times the store. Plus the add-ons for osCommerce are great. The paypal plugin is a breeze, so easy you're not even sure its working at first, the IPN support also make it hands down the best cart for digital sales also.
I have a cart setup at
http://www.hillbloom.com
if you wish to look then ask questions. Half of what you see on the opening page are addons.