Better to use 8 eneloops in the grip to get the 8fps. Unless you can get your hands on a BL-3, then an en-el4a will work too.

I haven't been shooting enough on my D300 to find out how many clicks will drain the 8 AA's and the internal battery, but I'm pretty sure you can fill at least 2 of those 8gb cards without swapping batteries.