I'd personally go with the P4B266.
I've got one, and I've taken my 1.6A to 2560MHz stably, and booted into Windows at above 2800MHz.
Here's why I'd recommend it over the SiS:
- Intel chipset + Intel chip = top-notch stability. The SiS chipset is more than likely just as good, but there's a certain amount of piece of mind involved.
- The i845 chipset is built on an 0.18u process and is in a flip-chip packaging; the SiS645 is on a 0.25u process in traditional BGA packaging. Why do you care? Well, the i845 chipset generates less heat and transfers that heat to the heatsink more efficiently. 170MHz FSB or higher is not out of the question on this board.
If you're only going to be overclocking to perhaps no higher than 150MHz FSB, the SiS board offers much higher bang for the buck and more flexibility in terms of RAM speed -- the P4B266 will only run ram at a 1:1 ratio at 133MHz or above. But for high-FSB overclocking, the i845 is unbeatable.