If you want the payment and registration at the same time, all the info gets captured in the same form on your site, including the card details, which is why you have to have SSL and serve that form with it. That card is not stored in your website, though, it's sent to Authorize.Net to process it.
Now, if you want to use buttons instead, like the Authorize.Net Simple Checkout seems to do, then you won't be able to do registration and payment in a single step. You may be able to have the person pay first and then register, as with the other gateway integrations s2Member Pro has.
s2Member doesn't integrate with Simple Checkout yet, and I don't know other people who used it, but you can read these threads and see if it helps with the integration:
Using gateways not in s2Member yet
viewtopic.php?f=36&t=3154Creating buttons outside s2Member
viewtopic.php?f=36&t=309#p1333