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This is based on the Canon EOS 40D Owner's Manual:


Recordingmedia: Type I or II CFcard


Like adrianholerga said, you can use a faster card (Type III or IV) but I don't think it will give you any advantages in-camera. The 50D does benefit from a "UDMA" CF card in the respect that it will increase the number of shots that can be taken in succession (high speed burst).
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