How much is ripple decentralised and p2p
Ripple is an internet protocol. It's ledger is kept on servers all over. In fact, I am in the process of installing RippleD and connecting my server to the broader network. 

Watch these short youtube videos: 

Here's an overview of Ripple ... 

Here is an explanation of Ripple in 60 seconds ...

Here are two more videos:

Here is a video to help you understand how a Gateway works ...  http://www.youtube.com/watch?v=M16ZatXbmLg

Here is a video to help you understand how the consensus ledger works ... http://www.youtube.com/watch?v=pj1QVb1vlC0

Then you should do a search on the Ripple Wiki and give it a read. (I'd send a link, but again, they're limiting me to two :-S ) 

Cheers!