extreme series is much faster than extreme III series.......its hard to beleive by the name but its the truth.....extreme cards have a transfer rate of 60mb/sec and extreme III series have 30 mb/sec......
and the rate varies a lot....extreme III cards are much cheaper than extreme series........ extreme III Compact Flash Card-16GB costs u around $150 while extreme Compact Flash Card-16 gb costs u around 230 $.........so i suggest go for extreme III series since the speed does not make much difference....