I recently purchased a K6-III+ 450 from Powerleap (powerleap.com) on the ebay site. Powerleap apparently purchased a number of the "+" series intending to buy the original K6-III. Powerleap sells upgrade kits and uses the original k6-III in their upgrade kits. The "+" series do not work. I have used it in my humble Presario 5220, replaced a k6-ii 380, and it works great. My mobo works with this chip. Many do not. I have been able to overclock it to 600 without even raising the core voltage (2.0) using these instructions that I downloaded from a person named UART. Heard about him via amdzone.com. Unfortunately, his website no longer exists, but the instruction set works great. In any case, you might keep an eye on the ebay site, or simply contact powerleap directly. They sent the chip very promptly, I was extremely pleased with them. Good luck.
Gigabyte P55A-UD4P, Core i7-860 Lynnfield at 3.6GHz, Zalman HSF, Corsair Vengeance (2 x 4GB), PowerColor HD5870, Earthworks 650W PSU, Kingston HyperX 120g SSD (W7 64-bit)
ASRock Z68 Extreme3, i5 2500K OCed at 4.5GHz, CM HSF, G. Skill Ripjaws 2 X 4G 2133, WD Black Caviar 1TB OS drive accelerated via RST by 60G Mushkin Enhanced Chronos, MSI GTX 560 Ti OC, PC Power and Cooling 610 Silencer, old Antec Super Lanboy case