I swear by
Cactushop....
I have used it on every e-commerce site I've built. It's so easy to configure, and amend and it's strikingly easy to dive into the code if you need to make heavy modification.
The support is first class, and it's admin area a snap. You can do just about everything you want with it, will integrate into just about any payment gateway... will support coupons, 'trade accounts' and you can set up affiliate systems so others can earn money from selling your stuff.
All in all, a cracking cart.