Somebody on the maemo-developers list figured out how to double/quadruple the speed of the mmc-interface.
He offers a downloadable kernel with double speed, for quadruple speed you have to edit the kernel-source yourself since he does not seem to consider this safe with any card.

http://maemo.org/pipermail/maemo-dev...er/005684.html

Tried it with my 1GB extremetech and 2GB Kingston and works fine :-)