If you go for DDR its best to get an AMD-760 based board rather than an ALi chipset board, which the KA266 uses, and can be SLOWER than a well tweaked SDR board and certainly more prone to problems.
I would say if you already have 640MB of 150Mhz capable RAM its probably worth going for the KK266 because this board will run 150 FSB no problem, giving a nice 300Mhz DDR on the CPU bus.