Kind of a neb questionFollow

Might have meant newb.

You'd might as well buy all of them once you hit your particular level so you won't have to spend more money in the future if you really need those spells.

Thanks. Why not more frost nova?
Also, Is amplify/dampen magic worth it?

because frost nova at rank 10 and rank 1 both freeze the mob for the same amount of time barring rank 10 does a bit more damage. Rank 1 also cost the LEAST mana. I seriously wished that BLizz just got rid of the other ranks to save us some time.

Amp and dampen is rather useful if you know where to use it.

Boss deals exclusively physical damage. Slap amp magic on tank let the healer heal tank for more.

Boss deals exclusively magic damage, slap dampen magic on everyone but tank if boss like aoe a lot. You will enjoy slapping the same spell on everyone in the raid in Kara (2 fights at least need this).

Detect magic, even if you do not have spellsteal which is a bleeding level 70 spell its still good to take it. The times that you would actually use it is rather little, I only really need it for the temporus fight in BM. It allows everyone to see buffs appearing on the boss. So whether or not you have been given dispelling/spellstealing duties its need for other despellers to know when to despell.

I use and apply dampen magic to a group, particularly with no healer. It helps when going up against casters.