Did some small research today on CF cards and the various specs you see on the packaging. I still don't trust speed specs on packages, and I bet many list max speed rather that average throughput so comparison in shops by looking at packaging info is almost impossible. But I think we can all agree that at least you buy what you need today and not for the future as prices drop really fast. What I found today is this thing called PIO modes that camera and cards somehow comply to. Depending on your camera, if you support an older PIO mode you may not be able to use the full speed of your CF card. Lets not talk about point and shoot cameras, 99% of them would probably do fine with the normal cards without the user knowing the difference.
Latest card like the Sandisk Extreme IV supports PIO mode 6, also known as UDMA in marketing-speak. Only the latest cameras support this card, like the D3 and D300 from Nikon. So, I take this to mean that you will not see close to 40mbps specced on Extreme IVs unless you use compatible cameras. The D200, D2X series and D2Hs supports PIO mode 6, so probably Extreme III will give close to IV performance. Got to check Rob Galbraith's database to be sure. Which my oldish D2H will find an Extreme III an overkill! However, still think Extreme IIIs are a good buy if I can find a fast card reader to offload the data. 4mpix pictures are small, only 3mb per picture, so 40mbps write is overkill.
Will be out looking for UDMA card reader. About time I spend less time waiting for photos to be downloaded.