I am not sure, though the back USB port is faster than the disc drive which means even a halfway decent USB 2.0 hard drive will be faster than a retail disc. As for SD classes, those only (officially) determine write speed and it is technically possible to have a Class 10 SD card with a read speed of one kilobyte per hour. That being said, in many cases the higher the class the better the read speed. 32GB Class 6 cards are fairly cheap nowadays and should be more than sufficient for running Gamecube games from (Gamecube USB loading is very slow and buggy anyway and is no longer being worked on). For Wii games, a USB hard drive is better due to larger capacity for the same price.
You cant just buy the fastest card around and expect it to write at those speeds. It depends on the card reader how fast it can write and read. I dont know the numbers for the Wii, but I suspect its not very high.
On the whole though, USB2.0 is faster than card readers, altough you cant really compare it like that.
A more correct comparison would be an external usb2.0 HDD vs a cardreader plugged into a USB2.0 port, in which case the HDD is almost always faster. But this too depends on a lot of other factors.
I'm looking for a benchmark of a class 10 SDHC card, to see if it's really worth it.
EDIT: according to these benchs they don't perform better than class 2 cards!
My conclusion: invest more in space rather than speed.