It only affects the read and write speeds, personally i try to go for the fastest i can afford but at the studio i work at we use Ultra II as shooting any faster than the buffer can write (even with a 30D and it's buffer - we use a 5d normally) will get bad shots because you dont give the flash heads time to reload.  You have to think what type of photography you are doing... i once did a test between extreme III and the ultra II (on my 30d) and the results are only noticeable when you do bursts and even then it is fairly minor like being able to take a 9 shot burst instead of an 8 shot (but that 9th shot could be the winner ).  So if you normally take your time over shots and arent trying to get a load of fast action shots then ultraII is fine i think. 

The other area is uploading the shots to your computer after the day is done and how fast the transfer is - for that i would probably recommend getting the sandisk reader... i have a packard bell one which is just bog standard and i dont see any reason to "upgrade" it does it's job and i am not pressed for time when uploading...

I've got my eye on a 4gb ex IV ... just because ;)

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography