I just had a chat with a friend who has some more experience with dealing with SD cards. He told me some unpleasant things about the potential write delays. It looks like I am going to need considerably more RAM as a result. Luckily the chip has a built-in memory controller with an SRAM mode that can deal with an 8 bit wide memory. I may have to switch to a different version of the uC to get enough pins, but that's not too bad. The physical size of the readily available non-BGA SRAMs is annoying, however.