Alternatively, you could use the Call of Cthulhu d20 system for magic and sanity. This system is mostly reprinted in Unearthed Arcana. A character's starting Sanity (and starting maximum Sanity) is based on their Wis x 5.
In Call of Cthulhu, magic is a dark and alien power. Whenever a spellcaster learns a new spell, he loses sanity. He also loses sanity when he casts spells. There are three levels of sanity loss, based on the type of campaign you want to run: Minor, Medium, Extreme (which I guess equates to how "dark and mysterious" you want magic to be in your campaign). The sanity loss for the spell cast is equal to the spell level for Minor, equal to 2 x spell level for Medium, or a number of d6 equal to the spell level for Extreme.
So, if I were running a campaign where magic causes a Medium loss of sanity and a character cast a 3rd level spell, he would lose 6 sanity.
Anytime the character loses sanity, he becomes temporarily insane (again, CoC and Unearthed Arcana have complete rules). Once you go to -10 sanity, your character is permanently insane. There are also sanity losses for seeing dead bodies, non-human creatures, demons, and other things that "just aren't right."
There's also an additional skill called Knowledge (Forbidden Lore), which was called Cthulhu Mythos in CoC d20. You can't take this skill at 1st level, but you acquire ranks in it anytime you read a dark tome of magic or learn something about the dark occult powers. Having ranks in Knowledge (Forbidden Lore) subtracts from your maximum sanity.
For example, I create Devon the Mage with a Wis 15. His starting Sanity is 15 x 5 = 75. His maximum Sanity is also 75. Along the way, he reads a dark tome of magic and acquires a Knowledge (Forbidden Lore) rank of 4. His maximum Sanity is now 71. Down the road he learns a 2nd level spell (losing 2 Sanity) and casts the spell three times (losing 6 Sanity total). His maximum Sanity is still 71, but now his current Sanity is 63.
If Devon gets healing for his Sanity from someone with the proper skill (probably Treat Injury or Knowledge [Behavioral Science] in d20 Modern], he can recover some Sanity, but never more the maximum. Basically, the character becomes more unhinged as he delves deeper into the mysteries of the Occult.
Lastly, there's an optional rule for "Getting Used to Awfulness" in the form of Sanity Resistance, which is some base number (3?) that increases by +1 per level. This is so that really high level characters get used to the sight of some of the terrible things they encounter.
Anyhow, these are some options for adapting Magic FX to Dark*Matter d20. I hope they write something up about it in Polyhedron sometime.