When you take a shot, the image is recorded by the camera then stored in the 'buffer' before it's written to the card. Writing to the card is slower than taking more photos in burst mode, especially at 8fps....so the images start to pile up in the buffer.
So, it's the size of the buffer that will be the determining factor as to how many shots you can take with an extended burst. Also, the file size matters...and you can usually shoot more JPEG images than RAW images.
Of course, the speed if the card does play a factor. A faster card will clear the buffer faster...but the buffers on modern cameras are so big, that unless you are using the camera like a machine gun, you shouldn't have a problem.
I just looked it up on dpreview and it said that the D300 was good for 100 JPEGs in continuous drive. I'm not sure if it would be less with the grip or not.
B.t.w....can anyone tell me why/how this camera shoots faster with the grip attached? This used to be the case with old film cameras, because the grip has a motor that would turn the film faster...but that's obviously not the case with digital. Does the grip just offer more batter power or does it also increase the buffer size or speed?