Yeah, but this brings up one of the core problems of 3rd edition and that's the eternal quest for higher stats and a general scorn of a stat that isn't +4 or higher. A +3 mod might be ok, but no one really wants to play a fighter that doesn't ideally have a +4 or higher.Eh... having it increase the score to a flat number means it'll be a lot more useful for some PCs than others (and not at all for anyone above 17). I think my idea will work well enough - as it is, they don't stack (they're both enhancement bonuses), and I see little reason to change the rules for this one case.
Back in 1e or 2e, you didn't have that problem. You didn't get a Str modifier to attack or damage until 16. And you just went with it unless you came across the gauntlets or a belt. Now, in a lot of ways, an 18 is an 'average' and if you don't have it, you're somehow deficient.
And 3e kind of encourages this with their array of stats allowing for an 18 or a short trip to get an 18, the point buy that lets you get an 18 or really close to it. D&D was never so stat focused in earlier editions.
That's why setting a fixed score seems like a bad idea. Plus, you're missing one of the most obvious advantages to a fixed stat! If your gauntlets fix your Str at 18, what happens when you're hit with a Ray of Enfeeblement that should drop your Str by 10 points? Nada, zip, zilch, nothing! Because the gauntlets keep your Str at 18!
Not such a bad idea when you look at it that way, eh?
Headband of Intellect? Oh, 18 not good enough? Well, you'll be praising that headband when you're hit with a Feeblemind and your Int remains at an 18 and not dropped to 1!
Starting to see some possibilities here yet?
Now, maybe you want an item to baseline your Str at 20 instead of 18. Fine.
Why not come up with a series of items that baseline stats at a fixed value (thus protecting the character from a specific drain or loss), while allowing them to stack with items that actually enhance the score? The item that fixes the stat value instead becomes like an inherent bonus or something, whatever different from enhancement so the two could stack.