Ext3 is a journaled FileSystem based on Ext2. It also has some additional features such as extents (which reduce the amount of overhead with storing where a file is stored on the disk for large files), and hash based lookups on directories solving the problem Ext2 has with large directories being very slow.
You can turn Ext2 FileSystems into Ext3 ones at any time by issuing
tune2fs -j /dev/partition
Note that unlike most disk tool commands, you add the journal to a currently mounted partition.
You can also add -J to pass journal options. The only one currently supported is size=xMB. See tune2fs(8) for min/max size.
Given Kernel support (eg CONFIG_EXT3_FS_POSIX_ACL in a 2.6.x Kernel), you can use POSIX ACLs by either adding acl to the mount options column in fstab(5) or setting a default mount option in the partition itself by issuing
tune2fs -o acl /dev/partition
Remember to change fstab(5) to mount the partition as Ext3, and then unmount/remount it or reboot. (You do not have to do this immediately). You can always mount an Ext3 partition as type Ext2 -- you just will not have any journalling performed.
You can also enabled hashed directories with a 2.6 kernel. This speeds up lookups for directories that contain a large numbers of files/directories.
tune2fs -o dir_index /dev/partition fsck.ext3 -D /dev/partition
The fsck is required to move the existing directories to the new format. If you ever remount the filesystem as ext2, directories that are written to will be converted back to the old format, so you need rerun the fsck.