It depends a lot on your shooting style and wallet.
Also consider the max size that your cam will take (some only do 2GB max).
If you are a slow thinker and take 1, 2 shots then recompose, think,.. etc, almost any card will do you. Expecially if you shoot JPG.
However if you shoot RAW and turn on machine gun mode and hold the shutter then you'll be looking for the fastest card you can get. Since you'll fill up the camera buffer ('pending on camera after 3/5/x shots shooting RAW) then have to wait for it to dump the data to card. This is when a faster card has advantages.
Extreme III or IV are damn fast and cost likewise.
I've a selection of Extreme III and others. A DaneElec 4GB card (cost me £17) is the bigest and slowest. I've a couple of Extreme III 2GB that get used most of the time.