Lesser art, also called Sorcery, draws upon internal force of the spellcaster, making him/her more fatigued, but is spontaneous, meaning that as long as you have enough stamina, you can keep casting any of your spells. These can be cast with different intensity or focus by becoming more fatigued. Some are petty and simple, others can be fairly powerful, but don't expect them to be battle-changing overall.
Then there's the Greater Art, also called Ritual Magic. These are the most powerful spells, but they are less versatile, with just a certain number of spells per level and magical school available each day. Normally you have far fewer of these, and you must carefully decide when and how to use them. However they are slower to cast; most take a full round, meaning you spend a full round vulnerable, and can be interrupted if harmed by enemies. Some greater Rituals can take multiple turns to be completed. These spells can virtually do anything in the highest levels: from making a meteor fall upon your enemies to summoning mighty undead or transforming you into a troll.
Damn! About all that is written above, I have been dreaming since the days of old fantasy RPGs. I hope you will be able to create this coolest system of magic!