I'm looking to purchase a Shopping Cart package that I can use to create E-Commerce sites for clients.

It needs to be secure, reliable, easy to use and update. I also need a package that allows fairly simple theme amendments.

Does anyone have any recommendations or advise?

Actinic has been around since essentially the beginning of the internet and I hear it's pretty good now:

http://www.actinic.co.uk/

I'd stay away from OSCommerce unless you're looking for a lot of work!

Gav