I'm using one 645 in my e-revo. It works just fine. I ran two for a while, but honestly I didn't find it necessary. I originally ran an 8xl, but it continually overheated and eventually I fried it. I've just finished installing a 9xl and I am liking that setup much better. Of course, I upgraded to an 8000mAh TrueRC pack at the same time, so I can't make runtime comparisons. It runs a whole lot cooler though.

The Revo's are a blast. I'm sure you'll love it.