Full frame or Crop Sensor Digital SLR?

The continuous frame rate for the 1Ds MKII was 4 fps, though I very rarely use this feature on any of my cameras. But others may well find it useful, and the D3's rate for its 12 meg RAW images is 9 fps, but needs a rest at around 20 pics to clear the buffer ( and an upgrade was later offered for those who wanted to pay for a larger buffer), but if shooting Jpegs, I think it runs to around 64 images continuous before it catches the buffer up, as the images are being processed and written to the card.

 I  find it amazing what's going on inside a Dslr at these sort of speeds taking photos; in the D3's case, 9 times every second it's checking the focus, the exposure, taking the pic and sending it on it's way to be processed.  If a DX lens was fitted ( now 5 meg pic sized), the rate went to 11 fps.

Its full frame stable mate, the D700 ran to 8 fps with the optional grip fitted, or 5 fps without.

My limited experience of sports photography (rowing, athletics etc) has led me to believe that I need a frame rate of at least 5 fps to get decent action shots. My old Canon EOS3 film SLR is capable of 7 fps with its battery booster grip attached. The big downside of  using it at this frame rate was that it could empty a whole 36 exposure film cassette in around 5 seconds !

With film, there were obviously no image processing issues for the camera to worry about. There was some limited processing for exposure and auto-focus, but the limitations of speed were largely mechanical in terms of mirror actuation and film transport.

As you say, with a digital camera, there is a great deal of data processing going on. With the professional DSLRs, the large image size and fast frame rate usually requires multiple processors to handle the data fast enough.