I'm guessing you've done it by now but...
I'm also a mac user and haven't needed windows for much, in fact only once. (will probably need it eventually to convert a few roms though)
The firmware update doesn't affect any of your rom, movie or picture files.
Installing Dingux/OpenDingux also doesn't affect anything for its being installed onto the SD card, just make sure you install the Dual-Boot files first.Dual-Boot
> http://code.google.com/p/dingux-code/downloads/detail?name=linuxspl.zip&can=2&qUpgrading the firmware
Dingoonity firmware > http://boards.dingoonity.org/dingoo-hacks/dingoonity-custom-firmware-1-2/
Read the instructions (Copy paste files to root, hold down while booting up)
This enables a few features including themes, the designer in me had to make a few adjustments...Dingux/OpenDingux
OpenDingux files > http://www.treewalker.org/opendingux/
Check your screen type first. Goto Settings > About. Press Up, Right, Down, Up, Right, Down - you'll see the LCD module number. Download the correct version.
Installing Dingux/OpenDingux was problematic.
I ordered a 16gb SD card and adapter, this worked fine could drag and drop files to it no problem.
The issue I ran into was booting Dingux/OpenDingux "FATAL: cannot mount /dev/mmcb1k0/dev/mmcb1k0p1 On /boot (17)"
OpenDingux wasn't reading/writing to the SD card properly.
Re-formatting the card on the Mac 100's of times and using 3rd party software done nothing - kept returning to the same error.
Opened up bootcamp/windows 7 formatted the SD card again - bingo. Worked first time, now using OpenDingux and waiting for a MAME release