For the arcane magic system I discussed earlier and the questions imposed:
How do they handle memorization, since it sounds like they still do?
An arcane caster learns spells through research, spellbooks, and scrolls. In order to memorize a spell an arcane caster must fulfill two qualifications. He must first be experienced enough to memorize the spell and second he must have the appropriate intelligence score.
For the experience part, you simply say that to cast such and such level spell you need to be level X. As for intelligence, you must have an intelligence of 10 + the spell level to memorize it as well.
Are they able to cast any spell they want, as long as they have the mana for it?
Yes, but they must meet the above qualifications for memorizing (level and intelligence). Plus they have to find the spell in a spellbook or scroll and be able to decipher the script. Then they must memorize, which works like this:
The length of time it takes to memorize a spell is 1 hour per spell level. The arcane caster must be in a quiet place free of distraction. Once the length of time has been met the arcane caster makes a Spellcraft DC check. The DC is equal to 10 + the spells level x 2. If the DC check is made the arcane caster has stored the spell into memory and is able to cast the spell. If the DC check is failed the arcane caster may spend the same amount of time studying and make another DC check, but increasing it by 1 point. If the arcane caster continues to fail and cannot make the check after 5 tries, he must wait until he increases a level to try again. A memorized spell may be written into the arcane casters spell book.
How does this interact with various magic items, like a Ring of Wizardry (presumably now just a mana source) or Ring of Spell Storing? How are metamagic feats implemented, as an additional cost to the spell?
Rings of Wizardry are mana sources. Each ring would provide a bonus to mana. The higher the ring the more mana it provides. It's a source of magic energy basically that spellcasters draw upon. It has a standard mana recovery rate.
A ring of spell storing would hold spells like it does now, but the it holds up to a certain amount of mana worth of spells.
Metamagic feats basically work the same. Where a feat that causes a spell to cast 4 levels higher now makes you spend the mana for a 4th level higher spell. You can use them whenever though. This can get very expensive. It's designed so a typical wizard will be able to cast his highest level spell 6 to 8 times, depending on his starting intelligence and if he continues to add points to his intelligence.
So like I said before, this wizard has a greater selection of spells, but has a lot total spells if he is casting his highest level of spells.
For instance, a 16th level wizard might have 136 mana, and can cast 8th level spells.
He could bust out (8), 8th level spells in a row, but that would leave him with 8 mana left... Generally a wizard doesn't sit there and cast his most potent spells, he uses the lower leveled ones.