Well, what those quotes seem to say is (approx nums):
1. the disk is 17GB of which
2. 200MB is /boot
3. 8GB is LVM split 2 ways:
3a. 2 GB swap
3b. 6 GB usable Linux ie '/'
4. 8.x GB unpartitioned
You'll need to use fdisk or parted to assign the rest of the disk (sda3) to a partition; type 8e (lvm).
Use the pvcreate to make it avail to lvm system.
vgextend vg_fedora12 /dev/sda3
lvextend -L +8G /dev/vg_fedora12/lv_root
resize2fs -p /dev/vg_fedora12/lv_root
NB: in order to avoid wasted space, use a much larger num in the lvextend cmd eg 10. This will tell you you don't have 10GB avail,but it will tell you how many extents avail. To avoid cvt extents to bytes, use the -l (lower L) switch with the num of extents from the error msg eg
lvextend -l <num-extents-avail> /dev/vg_fedora12/lv_root
See also http://tldp.org/HOWTO/LVM-HOWTO/
(NB: you have V2 of LVM).