got a few of these 4gb from Frys today. class 10? how high do they go? I should see a pretty good speed boost over the card shipped with the XM no? I need speed 
They are reasonable, but not really stellar performers:
http://www.sakoman.com/OMAP/microsd-card-perfomance-test-results.html
Steve
I think it should be much better than the shipped one because there is no class identification on it. So, the general assumption is "class 0".
The Kingston data in the table is from the Kingston that was shipped
with my xM. Mine was actually marked with Class 4, so if yours isn't
marked it probably is even worse performing than mine!
Steve