Ext3 is an extension of the Ext2 filesystem, currently adding journalling support.
where the only journal option currently supported is -J size=xMB. See tune2fs(8) for min/max size.
Note that unlike most disk tool commands, you add the journal to a currently mounted partition.
Also 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.
Ext3 is also gaining some features such as Extents (which reduces 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.
