I needed a new CF card for my camera, and even though it is a 300D and doesnt seem to support the speed improvements I bought a 120x CF card thinking that when I upgrade my camera later this year then i could get the speed out of it (and not have to replace the card having only had it a few months).
I was quite surprised to find that the camera writes to disk twice as fast as with my old cards (42x and 48x speed). I was not expecting the card to make any difference with the 300D. Apparently it does…
There’s a good list of card speeds in the 300D on Rob Galbraith’s site, here: http://www.robgalbraith.com/bins/multi_page.asp?cid=6007-6425
He’s done the same for most cameras, and it’s jolly useful - saved me wasting money on a Sandisk Extreme card for my 350D, as his site shows the improvement is only a couple of percent above that of the Ultra II cards.