I created 7 different 2500 mb partitions, each with a different filesystem, then i started to move some 500mb text files created with /dev/zero to the partitions, I was quite impressed by how much more hdd space ext2 and ext3 uses, specially if you use a lower bytes per inode number.

Partitions (2500mbs, 2.3gbs in Konqueror):

XFS - 1gb file = 44% used - 1.5gb file = 65% used - 2gb file = 87% used (324.5mb free)

JFS - 1gb file = 44% used - 1.5gb file = 65% used - 2gb file = 87% used (325.5mb free)

ReiserFS - 1gb file = 45% used - 1.5gb file = 66% used - 2gb file = 88% used (302.4mb free)

ext2 (default 1 inode per 4096 Bytes) - 1gb file = 50% used - 1.5gb file = 72% used - 2gb file = 94% used (140.6mb free)

ext3 (default 1 inode per 4096 Bytes) - 1gb file = 51% used - 1.5gb file = 74% used - 2gb file = 96% used (108.5mb free)

ext2 (1 inode per 2048 bytes) - 2gb file = 98% used

ext3 (1 inode per 1024 bytes) - 2gb file = 100% used

on another test I noticed that ReiserFS uses a little more RAM memory than ext2 and ext3, but i don't have the data for this test.