I wouldn't make either White or Black magic more powerful, at least at the levels the PCs can wield it (for example, the God of Good may be stronger than the Demon King, but their respective spell-slingers are on par). Instead, I'd make it a matter of how the magic is acquired... and at what cost.
Gray Magic is just plain ol' ordinary magic, in d20 terms mostly 0th- and 1st-level spells, going as high as 4th but with a shrinking spell list at each level.
Black Magic is the quick and easy path to power. It goes all the way to 9th level and it goes fast. However, it carries drawbacks equivalent to those in Call of Cthulu d20 (or something similar) - ability damage, madness, eventual descent into evil.
White Magic is the more difficult path to power, walked only by those who can resist the temptation to draw upon evil's readily offered might. It also goes all the way to 9th level, but it goes slow and steady.
At any time, any spellcaster could use black magic. It might even be for the greater good. But every time he does, the cost gets higher, and eventually he won't be able to stop...
Spells/day: Half a wizard's, rounded down, minimum 1 per available level.
However, any mage can cast any number of spells (or apply metamagic to his spells) if he's willing to risk the cost of using black magic, which scales with each daily use.
Many of the best spells for PCs are only available as white magic - like resurrection, heal, true seeing and hero's feast. Not only can these spells not be cast using black magic techniques, a character who has accrued a certain amount of taint from using black magic will find them unreliable at best, sometimes having the opposite effect (use the Pain of Healing rules from the Iron Kingdoms, if you have it)