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.
Depends on the cache chips if it is a "classic" Athlon. I have had a few with NEC 2.9 cache chips, and the one in my Primary system runs at 890MHz.
1/2 cache speed. The others have run well at 850-888 MHz, 1/2 cache. The ones that I have had with Motorolla cache chips topped out 800 or a little above/below.
At 1/3 cache with my Alpha HSF mounted on the core, I have had this one at a Gig, (700 with an 800 core) but prefer 890 1/2 cache.