Check out this Diablo 3 Bonus XP calculator made by bags+one.
"With the upcoming patch 1.08 a lot of stuff changes with regard to experience. It's not exactly straightforward anymore how bonus XP works since some of it is multiplicative and some is additive.
Even in 1.07, the best solution is not always "go max bonus XP gear", as the relative amount of XP varies a lot. I pointed this out in an earlier post.
To get an overview of what the max bonus XP in 1.08 is, what exactly the effect of certain items compared to monster power/multiplayer are, but also just because I wanted to play around a bit with Jquery, I created a little website that lets you calculate your bonus XP:"
Official forum topic.