Im thinking of getting another CF card. I am looking at the Sandisk website for answers to my questions and am coming up empty. What the heck is the difference between the Ultra IIs, the Extreme IIIs, and the Extreme IVs? Speed only? And how much of a difference is 10MB/s vs 20MB/s? Will I even notice? Id like to go on the cheap, so Im thinking Ultra II, unless someone says otherwise. I think the one Im using now is an Ultra II and it seems to work fine...