Sounds like you in for alot of fun..
Take it slow and use lots of lube (no personal experience just from what I've heard about anal sex in general). And if you end up liking it perhaps get her a strap on that has a vib built in or something.
*checks link*
Probably go the thin one first.. well.. that'd be my choice
and as Ambiguity said.. let us know how it goes
