Most relevant CF speed for a gripped D300

What is the quickest card speed that makes a difference for a D300 with 14-bit processing, RAW + basic JPG, MB-D10 grip w/ both batteries Nikon EN-EL3e?

In shooting fast-moving dogs playing fetch, I've managed to fill the buffer, at which point the shutter won't fire (the shutter won't go down when you press it).

So I'm looking for a quicker CF card.

There seem to be varying speed measurements, so I'll list all the ones I've come across:

30mb/s = 200x
60mb/s = 400x
90mb/s = 600x = UDMA 6

Wikipedia sez the "x" = the read time of a CD.

I've also seen the following, but don't have any idea how they translate, other than 6 is slowest, U-1 is the fastest:
Class 6
Class 10
U-1