I have my Asus A7M266-D sitting here in my office collecting dust. (Buyers keep falling through.)
It was/is a beast of a system especially if you put Barton mobiles in. It was my baby for a while, even after I had upgraded and bought a new system.
Anyway. Let's start off. What rev is the board? 1.03 or 1.04 etc? If I remember the 1.03 only let's you set the CPU voltage of ONE socket. SO even if you set the jumper for more voltage only 1 CPU gets it, the other get's whatever the CPU has for stock. Obviously this will limit your overclocking so check the board rev.
the MPX chipset on the Asus board never did as well as the Iwill offering. Pretty much 139-142 FSB was the common limit, although a few lucky individuals hit 150 it was rare. So 138 might be the wall. Realistically you are going to have to change the multi somehow. I used the pin mod/wire trick. The socket wire trick is reversable, but some argue in can effect heatsink contact, I never had a problem just so you know.
Like mentioned you can also convert those CPU's to mobile via bridges, and use a program like CyrstalCPUID to change multi, or whatever you like.
What I am saying is: If you have an older rev of the board the Vcore(CPU voltage) is only changed on one of the sockets, the other is stock. You have to modify the bridges on the CPU on the one that is the stock socket to get a higher voltage. If this doesn't apply to you, I'm also saying 138FSB is close to the limit for the Asus board with a MPX chipset(this limit is also dependent on you PCI cards). Other boards got closer to 150, and the Iwill was way higher, this Asus had a hard time, especially the earlier revs.
You are going to have to break down and change the multiplier if you want a higher clock, You may get a few more Mhz on the FSB but that's about it without changing that multi.
As for what CPU voltage? Well I'm not sure what you have, are they Palamino, Throughbred, or Barton? 1.85v won't fry them but I don't think more voltage is your limit right now(i.e. not the CPU), it's the MPX chipset or ECC memory at that speed.
I hate to recommend another forum, but search for a7m266-d at
http://www.2cpu.com forums, that's where alot of dually folks hangout that have overclocked this board.