To be frank, this is an example of the kind of nerf to spellcasters that doesn't understand why spellcasters are broken in 3.X derived games. If you do this, depending on the group, you are going to get one of two types of behavior:
1) You'll push your group to not play spellcasters at all.
2) You'll push your group to play spellcasters in a way that is even more broken.
Most of the most broken things that spellcasters can do in 3.X don't involve casting in combat. Of those that do, fireball is probably the least important.
What you are going to do is push spellcasters toward the model of buffing themselves and other party members which is far more broken in 3.X than fireball is. In fact, fireball is fair and generally balanced in 3.X. You are still in a 1e mindset if you think fireball has a balance issue. Instead of spellcasters doing easily countered and generally unbroken things like fireball or even (comparitively) finger of death, you'll encourage spellcasters to do scry/teleport/buff/called creatures - which is where the real brokenness lies.
It is quite possible to reign in spellcasters, but you have to know where the real problems lie. I've got no problems with spellcasters currently. If I do develop problems with spellcasters at high level, despite the changes, I have some general ideas what to do about it.
1) Ban spells. You'll need to do this at a somewhat blanket level. Ban everything not in the SRD, for example. Then add in on a case by case basis things that aren't in the SRD but which aren't problimatic, rewritting as necessary. This is far from a complete solution, as there are plenty of broken things in the SRD but it will avoid many of the more broken combinations and enablers you see. In particular, note how many of the 'cleric/druid/wizard has an answer for everything' threads involve spells from splatbooks.
2) Elimenate the Druid: The class can't be salvaged. Replace it with something more balanced, like the GR Shaman, and have the Shaman take the Druid's stuff. If you must retain the Druid, tone down the animal companion and elimenate the feat 'Natural Spell'.
3) Nerf the Cleric: The cleric is a well designed class, but a bit too powerful. In particular, by default the cleric should not know all cleric spells, but should have a list of spells known by level. Additionally, reduce the number of spells the cleric can cast per day by 1 per known spell level, so for example, the cleric starts being able to cast 0+1 1st level spells. The maximum number of spells can still be the same, you just get there more slowly.
4) Nerf Saving throw DC: I do this quite simply. Don't add the spell level of the spell to the DC to resist the spell. This in one stroke greatly reduces the problem of 'save or suck'. Once you start carefully removing any of the broken ways to boost DC of saves unreasonably, you are well on your way.
5a) Fix the brokenness: This is the last and to some extent hardest step. It's so complex, I've broken it into two stages. Fixing the broken stuff, and restoring spotlight to non-casters. To fix the outright brokenness, you have to fix all the following: called creatures, all shapechanging spells, any save or suck whose effect is out of scale, no save and suck spells (3.5 Blasphemy, I'm looking at you), and any spell that offers no conventional counter (forcecage and detect evil, for example, cannot be reasonably defeated either by attacks, skills, or abilities but require either magic or highly specialized class abilities). In general, a good place to start is to look at the 3.0 SRD and compare it to the 3.5 SRD and take the weaker of the two versions of the spell. For example, 3.0 Alter Self is much less broken than 3.5 Alter Self. Another good place to look is the sort of changes between spells in 3e and 1e, notably things like Polymorph Self and Polymorph Other, where balancing conventions in 1e were removed resulting in outright brokenness.
5b) Give No Absolute Skill for Free: Be on the watch for spells that have unquantified effects or which are available to low level casters and replace very high skill with an easily available power. A good example is 'Zone of Truth', which gives you effectively better than +100 Sense Motive. Those spells need to be tied to the skill system, or given small quantified effects, or have their level increased. For example, fly is almost infinitely superior to balance, climb, and jump yet is available at 3rd level in a very unrestricted and ideal form. This suggests fly is too low of level. Invisibility is another candidate. You can replace these idealized forms with weaker versions - winged, low stability, average manueverability flight - or invisibility as a phantasm (effecting only a few targets rather than all observers), when you bump up their level. Any spell that lets you no or do something that would normally require a skill check, should require a skill check. Any spell that grants skill at something should probably not grant more than a temporary +5 or +10 bonus at most. One thing that serves me well is bringing back the 3.0 Scry skill and forcing skill replacement to at least involve a skill check. This ensures that characters without spells arent' inherently worse than those with spells.
Once you do that, balance is largely restored, and certainly will be restored through the traditional D&D 'sweet spot' ending around 13th level or so. To fix the rest, you have to make other changes - many of which don't involve spellcasters.
One word of warning, if you do to this without toning down in some similar way the rest of 3.5 or Pathfinder, you'll end up simply making spellcasters suck. In general, both 3.5 and Pathfinder have tried to deal with balance issues not by meeting in the middle - making spellcasters less powerful and non-spellcasters more powerful - but by simply ramping up the power level.
