I haven't looked too deeply, beyond searching out a Micro SD card for my Windows Phone 7 device. I remember that the higher class cards tended to have worse random access speeds, and it was recommended to use a lower class card in the phone. This was due to it striping both the internal flash memory and SD memory into one big usable chunk. If the SD card didn't perform well with random read/write access, the OS could slow down, or even crash.

As for the ExpressCard reader showing up as a USB 2 device, that is probably normal. ExpressCards can tap either into the PCIe bus, or the USB 2 bus. Most card readers I saw were of the USB variety, due to the manufacturer just throwing the same guts into both ExpressCard and USB pluggable form factors.