Can I use the practices in isolation and what are the risks of doing this?
Yes, you can use any practice on its own, but the inherent weakness in that practice needs to be mitigated.
What is the best practice to start with today?
It depends on where you are in your lifecycle. Some teams have changed project approaches midstream and used the XP programming practices inside the context of an existing structure. Developers should try unit-testing tools that can be used across development styles as soon as possible.
Where can I find some more facts about the costs and benefits of pair programming?
The home of pair programming is http://www.pairprogramming.com. There is an excellent article on the costs and benefits of pair programming at http://collaboration.csc.ncsu.edu/laurie/Papers/XPSardinia.PDF.
Is XP simply the name given to the 12 practices? What is the glue that holds it together?
No. We started learning about XP by looking at the values and how it overcomes common development challenges; it's these values that are the "soul" of XP.