OK, I've looked at the manual and the unofficial FAQ ( http://web.archive.org/web/20010823080228/home.socal.rr... ). I've tried to contact IWill (no answer, so far) and I've posted to various forums. I keep getting conflicting answers to this question and so far I don't trust any of the answers because none of them sounds official enough.
Everyone agrees the Thoroughbred core is not supported at any frequency and that the T-Bird is definitely supported at all frequencies. (It only goes up to 1.4 GHz.) There is disagreement about the Palomino core and the frequency--if any--that is supported.
A guy at Hard|OCP said he owns one of these boards and even with the latest BIOS he couldn't get anything over a 1.4 GHz chip to work. Period. Someone else online said a 2100+ Palomino (1.53 GHz) works just fine (but he doesn't actually own one of the motherboards, so I'm disinclined to trust his opinion). A 2200+ won't work because anything above 2100+ is a Thoroughbred. To complicate the issue, there are both Palominos and Thoroughbreds at 1700+ to 2100+. Anything from 1600+ and below is a Palomino. (This is from memory, so don't quote me, please. I'd have to look it all up again to make absolutely certain.)
If the guy at Hard|OCP is right, I can either get a 1.4 T-Bird or a 1600+ Palomino that runs at 1.4. Then again, I wonder if he had an adequate power supply and heatsink/fan combo. I also wonder if he remembered to adjust the core voltage when he attempted to run a Palomino on this board. I guess I could try PMing him to see what steps he took.
The old unofficial FAQ had this to say:
Iwill's new Socket A motherboards, KK266-R and KK266, features the much-anticipated Apollo KT133A north bridge and VT82C686B south bridge from VIA Technologies Inc. Ultimate Upgrade features include: support for 266MHz Front Side Bus (FSB) Athlons coupled with 3 Phase Power to support up to 1.5GHz.
Then, under CPU's:
* What CPUs' are Compatible? Your KK266 is designed to be compatible with both the "B" type (100/200mhz) and "C" type (133/266 mhz) AMD socket A(aka Socket 462) Durons and Athlons(Tbird)CPUs. The "B" type is "sold" to run at the 100 FSB (front side bus) speed. The "C" type is "sold" to run at 133 FSB. While the board runs at 100 or 133 FSB, the CPU runs at 200 or 266 due to it's EV6 DDR architecture. Your KK266 allows you to run either CPU type at either FSB speed thanks to its mulitplier (frequency) adjustments and FSB adjustments. However, you can't run your "B" type at 266 without some modification. See the Overclocking section. It's up to you to unlock the CPU's multiplier by connecting the L1 bridges if you want to overclock it.
*ID your CPU: What's the meaning of all those numbers and letters on top of your CPU's core? OCinside shows it all: Vcore, the Case temp rating, the L2 cache, FSB speed (type "B" is 200mhz, type "C" is 266 mhz), the year and the month of manufacture, etc.
*1.4 GHz Athlons:
It just leaves you hanging after the colon and doesn't say what kinds of 1.4 Athlons are supported. I'm assuming the T-Bird and the Palomino, but not the Thoroughbred (although I'm still not completely sure about the Palomino).
Some overclocking information (a link to another site) mentioned speeds of up to 1.53 GHz. Color me confused.
I don't know what the real answer is. I can't find anyone who sounds "official" enough. I've tried contacting IWill. Once I got what looked like a 404 Error, but it was in Chinese. Today I tried regular e-mail. Hopefully, someone will get back to me with an "official" answer.
Until then, I'm hoping someone here has hands-on experience with the old IWill KK266.