@Psymon: Don't be afraid, I had something differen in mind. Let me explain:
in BG(and other Forgotten Realms based games) spells are ascribed to one of levels from 1 to 9 with level one containing simple spells and level 9 with most powerful ones. A magic user could cast only limited number of spells(no mana cost here) before resting. Number of spells a character could use(and from which level) was determined by his experience level and his intelligence. With experience the character was gaining access to higher level spells. Number of spells available to cast on each spellcasting level was dependant on character level, which means that characters on level 5 sould cast e.g. 6 spells from level 1, 3-4 spells from level 2 and 1-2 spells on level 3. Upon resting the character could cast the spells again.
This system might sound a bit weird for someone who didn't play Baldurs Gate or Neverwinter Nights, but it has one adventage: it impells to strategic use of your magic.
I plan to make it that way(it's an optional solution at the moment):
all spells would belong to one of levels from 1 to 5. PC would gain access to greater number of spells and higher level spells with experience(+some modification from intelligence). The spells will cost mana, but PC(well, the same goes to NPC's) will be limited to number of times he can use those spells before resting. This will make use of this magic a bit more strategic.
As I have mentioned before, to be able to use a spell, one has to have apropriate skills. Most of the spells will have it's effects dependant from the caster's skill(so, if Fire Burst requires destruction on level 75, it will do more damage if caster has destruction on level 100, or even more if on level 125).
@Sulteric Drums: thanks for the proposition. Soon I'll need testers, so I'll remember that.
brucevayne