I had an addon that helped with that. Basically it used dogtags and I told it that when my mana is >= Xnumber to show the icon for mana gem, etc. I will see if I can find the name of it.
sigh I haven't used it for a year and can't find it.
edit #2 -- Found it!!! It was called Underhood (or Underhood2)
Edited, Nov 10th 2010 8:22am by Anobix
On a side note, this looks pretty awesome for an easy-mode arcane 'rotation'.
http://www.youtube.com/watch?v=lO_6pBDHOU8
http://noiretendard.free.fr/wow_addons/Raven_plugins/Mage.html
Edited, Nov 10th 2010 8:45am by Anobix