Kind of forgot about this so I thought I would just post an update. I was using the standard SD card library at the time to log data taking 0.3s for each iteration, a speed I could barely handle, and 0.6 would be catastrophic. In the meantime I have done a lot of development, and after I came across this thread, http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1293975555
, I have been able to write an iteration of data in less than 30ms, meaning I could probably go down to 8mhz by now.
I am not sure whether or not the sd card is expecting data at a "proper" baud rate, but my limited knowledge would make me guess "no", as I assume the clock signal comes from the arduino - But I haven't looked into detail about that. I would assume I would have to look into the SPI protocol to find out. In general, serial would be nice (to monitor when debugging) but is not a must