I believe the basic requirements of an online lvm resize is kernel 2.6 or later AND an ext3 file system. Since your using the latest centos 5.5 then you must have kernel 2.6.18-x and hopefully an ext3 system.
This is how I would do it:
1. create a partition on the second hard drive.
2. declare it as a pv
3. extend the existing volume group of the logical volume you want to extend by adding the pv you just created.
4. #vgdisplay -v <you vg name> to check if the vg has really been increased and by the right amount too.
5. now to resize the logical volume: (note: change the values accordingly- don't forget!)
# lvdisplay <--- check the current 'LV size'
# lvresize -L 386MB /dev/volgroup001/logvolvar1 <--- you set what you want the size to become not how much you want to add.
# lvdisplay again
# df -h /dev/mapper/volgroup001-logvolvar1 <--- your should still see the old size.
# resize2fs -f -p /dev/volgroup001/logvolvar1
-f = force
-p = print summary of progress
# df -h /dev/mapper/volgroup001-logvolvar1 <--- you should see the new size
You don't have to unmount the fs; but it is better to be on the safe side and do the above when there is minimum or no program accessing the fs.