Automatic partitioning is safe and fast for standard installs—choose it if unsure. Manual partitioning is needed if you dual-boot, use LVM, or want separate filesystems for different partitions. Plan ...
I've assisted a friend establish a dual booting Windows/Fedora3 system and after the install, he asked me if I could resize the Linux partition from it's current 80gb ext3 to a 40gb ext3 and 40gb vfat ...