How about:
No mana
Long term stamina - for all actions
Mental stamina (short term) - for magic
Physical stamina (short term) - for physical activities
However, if we consider drawing magical energy WHEN we cast the spell, then the power-up bar that is in existence now would need to be replaced by two: one for faster casting and one for stronger casting of spells (as either would be possible).
Let me explain my idea: Both bars are at 100% by default and they go from 0% to 200%. They are now called spell speed and spell power - I think they should be part of the spellbook window instead of stats. Now, you may toggle these two at will, but the percentage total of the two must be equal to or less than 200, or else you can expect something bad to happen. If you are lucky (and haven\'t pushed the bars to high), then perhaps you might only experience spell failure or a minor backlash from time to time, but beware if you get to greedy, though...
IMO, if you give it a thought, that would be as realistic as this system could get.
Now on concentration - there could be a bar like this: pull it above 0 (default level) and experience benefits, whatever you may be doing. The bar keeps going down by itself all the time and burns a lot of mental stamina while above 0.
On the other hand, I would dismiss the idea myself as being able to concentrate on each skill in particular should be a part of your skill level (education in that skill). Except perhaps if it would count as \"trying hard\" or something along these lines.
