Anything.
The 6.5 fps burst speed is dependant only on the camera's buffer. Once that's full the rate will slow. The 40D can, IIRC, only handle around 11 MB/s - so any card faster than that will be more than the camera can make use of.
The AAAx ratings of cards dates back to the neosilica age when the same rating was used for CD drives. 150 kB/s = 1x, so 100x = 15 MB/s which is the best your camera can handle.
NB. Please, please, use the correct units. It's MB/s - for MegaBytes per Second. mb/s means millibits per second. That's 8000 times slower.