You can still access data/music/videos if your SDCard is formatted in FAT32.
As for which you should go with, that's upto you. If you are running an older ROM (<2.*) - you may want to think about partitioning your SD for ext2/3/4 (where your apps will be) and then FAT32.
If you are on a newer ROM (>1.6), you can format it as FAT32 and utilize the entire SD card.
(If you have any performance issues, you can always download Swapper and create swap-space for your phone to use, but you should be fine if you are running a Galaxy)
Phone: Samsung Galaxy S 4G / T-Mobile G1
ROM: G1: CyanogenMod v6 (Froyo)
Recovery: RA Recovery Menu 1.7.0
SD: 8GB Class 6 microSD
Hacks: 10 MB Hack
Plan A is always more effective when the device you are working on understands that Plan B involves a large hammer.