After checking out several spell point systems we elected to go with the Mana Spell Point System by Gaetan Viau, making a few modifications to accomidate our damage system. The system is designed for Mage and Bard classes but we have found that it also works well for priests and druids. Here is a brief run-down of how the system works. This is not the complete system...if you would like the complete system you can get a copy from the netbook archive.
Mana spell points represents the ability of a spellcaster to cast spells; the amount of mind strength that this spellcaster has to retain the energy present in his surroundings. This energy is needed to release the desired magical effects. The number of Mana Points a spellcaster can retain is directly related to his/her experience level and Intelligence score.
*Use the mage table for the illusionist, priest and druid.
The spellcaster also gets bonus Mana Points due to high Intelligence score. These bonuses are cumulative for every experience level attained up to the 10th level inclusively.
When the spellcaster casts magical spells, he expends energy that his retained in his mind: Mana Points. The amount of Mana Points used to cast a certain spell is related to the power of the spell (its level). Specialist wizards dedicated to a particular school of magic are more attuned to the spells from this school and thus require less Mana Points to invoke the power of those spells. The SCOST is then used by specialist wizard casting spells from their speciality school instead of the COST. Please note that the bard never uses the SCOST.
Every time a spell is cast, the COST (or possibly the SCOST in the case of a specialist wizard with speciality spells) is reduced from the total remaining MP the spellcaster possesses. The wizard can (and note, ONLY the wizard since the bard lacks the spellcasting ability to do so) if he wants, reach a negative MP total. For him to be able to do so, he must successfully make an Intelligence check. If successful, the COST of the spell is first subtracted from the remaining MP total of the spellcaster. Then, every MP reached below zero will result in the loss of one box per body area of the Wizard. If the Intelligence check is failed, the spellcaster loses an amount of MP equal to the spell level + 1, the spell is lost and has no effect. Likewise, every MP reached below zero will result in one box per body area lost by the Wizard. This loss is counted as wound damage but represents fatigue and can be recovered with a good nights sleep.
After the spell has been cast, the Wizard needs to make a System Shock roll adjusted negatively by the number of points lost in this manner. This roll is made at the very end of the round in which the spell has been cast (whether the spell has a duration or not is irrelevant). A wizard failing to successfully make this roll, falls unconscious for 1d6 turns (unless of course the loss of points killed him). This loss of boxes can only be healed by rest or by the use of a heal spell or a limited wish (any cure spell won't work).
Needless to say that trying this little feat for a wizard with a low Constitution score can be very hazardous.
For additional information on the Mana Spell Point System please download and consult the Netbook.