T-bird, or classic? (IE, 256k ondie L2 cache, or 512k offdie L2 cache?).
If its a classic, I'd say about 850 or so would be tops (without adjusting the multiplier), I got my classic up to 863 (2/5 cache) with a 123mhz FSB on my KA7 (VERY RARE TO GET That),but with a gold finger device, it shouldn't be a problem.
T-birds, well, those are a different story. Slot A, or socket A? Slot A's don't usually go above about 850 from what I've heard. But then again, its partially because of the timing issues on the KX-133 chipset with slot A thunderbirds.
I guess I would say that you're likely to get at least 800mhz out of it with good cooling, maybe more if you go to more extreme measures.