The latest version of the A7M266 is Version 1006, in which I'm running right now. I'm currently using an AMD Athlon XP 1900+ and with Version 1006, the Motherboard *should* support Processors up to 1800 MHz in "Real" Clock Speed.
Although there is a Version 1005 available, I highly recommend that you get Version 1006, but it's your personal opinion that does all the decision-making, but having a newer version of the firmware can't hurt either.