Well tbdbuckeyeitl I can tell you first hand that after making my first purchase, every subsequent purchase had my credit card info already filled in and saved, and showed the last 4 digits. All I had to do was find an app, click Buy, then wait for it to load, click Purchase or whatever underneath the price and which card is selected, and then install and boom. Purchase made just like that.
So yes, it does save your info. I'm sure there's a way to punch it in and not save it, just haven't looked into it because I am anal-tight with my phone and don't really worry about this. It just saves me time from punching it in everytime.
As for the first time you buy an app, it asks you for your name, address etc etc, then you punch in your card info and you continue on your merry way. I can't say what happens if you already have an account, because I didn't have one when I got my X lol