The Extreme I, II, III isn't quite as simple I have 3 extreme cards 2x III's and an Extreme (no number) which is a class 10 card the other 2 Im not 100% sure what the difference is, both say class 6 but 1 says 30MB/s edition which is what the class 10 claims to be.
Unless you're shooting LONG bursts in RAW at high frame rate (or HD video) then it doesn't really matter.
FWIW I always use Kingston memory cards - good provenance (from their computer memory history), good speed, come with a lifetime warranty and are cheaper than Sandisk.
My advice is......... Get two!
If one fails, you always have the second one.
If you haven't got a second one, you are stuck
6 x 8gb 2 x 4gb abd 4 x 2g all at least rated 6 and either Kingston or Sandisk, if you use 7 day Shop buy via Quidco for extra discount.
I am sure the other well known brands are reliable, but TBH I stick with what I have found reliable and works for me without any problems, thats Sandisk.
8GB Extreme CF card prices today:
Amazon £37.18
Play.com £39.99
7Day Shop £43.99
All including delivery

Nothing regarding brand but don't buy it on ebay is the best advice, too many people selling fakes on there.
7dayshop are pretty much the cheapest I found for the high speed sandisk cards, buy multiple smaller cards rather than a single large one is also good advice because even the decent cards fail occasionally and you tend to save a bit as most people would prefer to buy 1x8GB over 2x4GB as a hobby thing your are pushing it to fill 4GB in a normal days shooting jpeg. example the 7dayshop 4GB extreme are £18.99 the 8GB version is £39.99 I know its only a couple of quid saved but I would rather loose 4GB of photos than loose 8GB of photos if a card fails, gets lost or damaged and I can stick the other card in and keep shooting rather than that being the end of the day.
