jCart is a free PHP / jQuery powered cart but you would have to integrate your own payment provider, database etc. into it.
This type of functionality is something that I have been planning to look at for a while for my clients but I am just to busy at the moment to develop and refine it.
WA *may* build something like this into Powerstore eventually, they already have some jQuery powered stuff in the admin, but I cannot see it making it's way into eCart as part of the wizard at the moment.
Views expressed are my own. yadda yadda