Even with the newest Canon DSLRs (i.e. 40D, 1DmkIII), they don't support the maximum speed of the Extreme IV cards by Sandisk.
If you need the maximum speed in-camera, you should just go for the Extreme III flavour. If you don't need the highest speed, I would go for something like the Ultra II range.
I would avoid the cheapest/slowest cards because they don't come with good warranties and recovery software in the box like the posher versions.
Also... make sure you don't source your memory cards from ebay - there are tonnes of fake cards on there and some of the fakes don't even have the advertised capacity let alone speed of the genuine articles.
I've got an Extreme IV 4GB (and a FireWire 800 reader for it) but it's just because I'm an impatient git when it comes to getting the pics onto my computer...
