shoping carts

Not sure where to post this question but I thought Id try here.
I just had an old client call me and wants to switch their shopping cart provider. They may even want me to stay on and manage the whole thing for them. Any advice on a reputable ecomm/shopping cart provider and how I may broker this deal?

I use Magento - it's got all the features that I want (and more).

It's full e-commerce though, not sure if that is what you were after?

Is the site made on any platform such as Joomla or Wordpress?
Id advise Virtuemart for Joomla sites

It is a e comm site. Full shopping cart thousands of products. They are a supply company. I will check to see what type of site. It was built through the company thats hosting it all now, which they are really not happy with.

Interspire, I never use any other type of cart! It's easy for clients to use and very SEF!

So far I have 5 that I am looking at possibly including the suggestions you guys have contributed(thank you btw). In no particular order they are; Volusion,Advanceware,Prostores,magneto and Interspire. Any further knowledge or advice is welcome. Posting any personal opinions on pros vs cons would be appreciated. This could possibly turn into a rather big account(for me) as they are a national supply co, despite the owner crying poor all the time. They called me back again for a reason and as usual plan on delivering the best I can the best way I know how. Any more input would be great, I will keep you posted on how this goes and if and how I propose the deal.