the dc's we use are 10 + level of the spell + ability modifier +1 for every 3 levels of the caster.
My concern about this is that it means that DCs at 20th level casting 9th level spells with a grey elf wizard who started with 20 int and put all his points into int means that they have DCs of 32. The average 20th level fighter with no magic items has +15 to Fort saves, +7 to Reflex saves, and +6 to Will saves(each of them would be 5 higher if you are allowing cloaks of resistance).
This means that if there is a save or die spell that is 9th level that targets them they likely need a natural 20 to succeed. If it targets Fort then they need a 17 or higher to save against it.
Contrast this to the 2e save system where at 20th level as a fighter you need a 3 to save against all death magic spells.
Plus, a 20th level fighter in 2e would need a 6 to save against a fireball. The caster listed above means the fighter needs a 19 in this system.
If you want wizards to be as powerful as they were in 2e, you need to factor in the fact that at that high a level, most of their spells had no effect at all due to the ease of making saving throws. Using my method, the above fighter would need a 14 to save against the fireball and a 6 to save against a Fort save or die spell. Which is much closer to the 6 and 3 respectively that they'd need in 2e. With magic items, this probably balances out to about the same.
However, I'd still suggest the doubling of all damage rather than a geometric progression. Hitpoints are not geometric. They go up faster than that due to con bonuses. They WERE geometric in 1/2e, but that's actually what causes the direct damage to hitpoint problem in 3e(in addition to toughness, improved toughness, and max hitpoints at first level). If a creature has a Con of 12 at 1st level and uses d8s for hitpoints, it will have about 31 hitpoints at 5th level but 64 at 10th level likely. Using a purely geometric progression would mean that they'd only have 54. Monsters in 3e either have higher Con bonuses the higher CR they are, or they just have that many more hitdice, which increases the effectiveness of the Con bonus they already have.
If the idea is to have a spell do about the same percentage of a monster's hitpoints as it did in 1/2e, then you need to account for that bonus. When I lost the details in the previous post, it had basically said that hitpoints had increased amongst classes by between 1.5 to 2 times. I reasoned that if you doubled the damage on a fireball that did the same damage that it did in 1/2e, then you'd end up with a fireball that was just as dangerous as it was in previous editions. But only if you keep saving throws about the same as they were in previous editions to make sure monsters took half damage about as often.
It works slightly better than using 1d6+3 per level since in 1e/2e, it was certainly possible to roll 10 damage against a creature at level 10 with a fireball that did negligible damage to it. Adding a static number guarantees damage a little too much. It means that the minimum damage is 40. Which is enough to kill a 10th level Wizard without a Con bonus. Versus minimum 20 damage with the double dice method which won't even knock said Wizard unconscious. Which is closer to the "feel" of spells from 1/2e.
Other than the doubling of damage and changing the DC of spells, most of the spells function very similar to the way they did in 2e. I, personally, wouldn't change them almost at all other than this.