The A7M266 was, if I remember correctly, the **second** DDR mobo available (next to the Iwill KA266).
You can try an XP but don't be surprised if it doesn't. That chart from the german Asus site makes it look like Palomino XP's are supported. If you already have this board I'd recommend flashing it up to the newest BIOS and maybe borrowing an XP CPU from a friend to test.
I already have an XP1600 to test in it, I was asking here just to check first. I am most likely going to get the board regardless and if the 1600 doesnt work it will be a descent board for a second sytem anyhow.