I don't have this game but i'm getting the feeling that I want it, so I wanted to ask if you can take a mage to be an Archmage, or is there a mod or something out there that allows that?

Thereare no prestige classes for any class. However sure you can all the way to level 20 if thats what you mean. You will also get there probably far before the end of the game.

Mage is a pretty good class as well but be sure to be careful with your skill points.

I read in a post by Volguuz RageWaar (topic: sub class?????) that if you place the name "Arch" in the sub-race category when you create your character it will appear that you are an "Arch Mage" on your character record...try it and see.