Yup. The camera has a buffer of very high speed memory, and different cameras have different sizes. In fact, if you go to the expensive bodies, one of the main things they have is more buffer memory.

When you take a shot, it's copied very rapidly from the sensor to the buffer, and as soon as the data is out of the sensor, the camera can take another shot. As soon as there's data in the buffer, the camera starts copying it onto the relatively slow compact flash card (or whatever your camera uses). This can happen in parallel with taking new photos.

If you just sit on the trigger, your images will be taken one after the othre as fast as the camera can unload them into the buffer, but since the buffer is emptying more slowly into the CF card than it's filling, it will eventually fill up, and you can't take any more photos until enough buffer space has cleared to hold at least one more photo.

The smaller the images, obviously, the longer it'll take for the buffer to fill.

So think of the buffer as a leaky bit bucket whose bits "leak" into the CF card at a constant rate. When you take a photo, the entire frame of raw data is dumped into the camera's converter to make it into jpeg or whatever, and the jpeg result is dumped into the leaky bucket. Since the whole raw image comes out of the sensor, it can't take photos faster than 2.5 fps in your case, and the conversion to jpeg makes those images considerably smaller.

So taking photos at full speed, each frame dumps a load into the leaky bucket, but the simpler the images are, the smaller those loads are. If the
camera had a file format that was so small and simple that at a 2.5 fps rate the data in was no larger than the amount that leaks to the CF in the fame time, then the camera could take images forever -- well -- until the CF card filled up.

If you're saving as giant RAW files, then each frame time you're tossing in a huge pile of bits into the leaky bucket, and it doesn't take too many of those to fill the bucket.

I would recommend against the SanDisk brand from my personal experience with them. Besides, Lexar is what works best with Nikon as that is what they designed around. I have also had very good luck with Promaster/Delkin cards.