To fix the situation, you have to understand the situation. When people talk about "overpowered casters," odds are they're talking about the Big Five:Archivist, Artificer, Cleric, Druid, Erudite, Wizard (I know that's six). If you cull those classes, there are plenty of classes to replace them.
The basic problem with most of the Big Five is that they essentially have an infinite spell list to draw from, allowing them nigh boundless power if you use those spells to full effect. Sorcerers are generally fine because they have X spells, period, and they cann't change into the perfect character for the day every morning, or pull out the perfect ace every time. The Big Five can. When you're talking about Beguilers, Dread Necromancers, Wu Jen, Shugenja, Ardents, Psions, Warmages, Warlocks, you've really brought casters more into the realm of sanity.
Then, all that remains is to fix melee.
Now, if you're trying to balance the Wizard against the Fighter, stop right there. The Fighter (and perhaps the Monk moreso) is hideously underpowered. Just throw out the pure Fighter (and most other pure melee classes pre-ToB) in favor of Warblade and you just pretty much fixed melee, bringing them in line with the reasonable casters.
So, in short? The caster fix does not involve nerfing casters. It's to cut out the worst offenders and bring in Tome of Battle for melee.