It works fine on the KK266/KK266-R. The core is close enough to a regular Thunderbird core in design that it should run on almost all Socket A boards. You might want to shoot Compuwiz1 or TuffGuy a PM as they both have used Athlon 4/MP's on the KK266 with success. The only real issue is if the bios of the motherboard they run on has full support for all the new features like data prefetch and SSE. The Athlon 4/Palomino core was designed with DDR in mind because the data prefetch better utilizes the extra bandwith that a DDR system will provide. However, it should still work with SDRAM, just to a lesser degree. It can also benenfit from higher FSB's that boards like the KK266 can achieve.