a factory reset won't touch your SD card. i had to do one on my phone because it just up and died and rebooted itself into a boot loop. all a factory reset does is format your /data partition so all your apps and home screen settings go bye bye.
Samsung Galaxy Precedent (rooted, AreaRom 3.4 , sd-ext w/ 3.65GB internal space)
Kindle Fire (1st Gen, rooted, AOKP JB 4.1, Nova Launcher)
Samsung Galaxy SIII 4G LTE (rooted, Carbon ROM)