My E8500 ran at 1.225 with stock settings according to my bios, I found it would run stable with lesser voltage, for my overclock I used 1.2675.
But these figures are meaningless to you, you have to find out for yourself what you need.
First find out your vcore with cpu-z , stock settings ofcourse. Now you know where to set the vcore in bios when you set the cpu fsb higher.
You could also set 1.30v and set the fsb higher in steps.
I would guess you could do 9x400 at approx 1.3v, could need more, could need less.
You just have to find out for yourself, with the aid of cpu-z, realtemp and prime95.
I just bumped the cpu fsb to 400, set memory strap so it would do 801Mhz (ddr2-6400 mem) left my multi at 9 ( E8500 has 9,5 multi) and started to raise the vcore in small steps. First some noboots, then some crashes while running prime, eventually I found a stable vcore. When I set my multi at max I needed to bump the vcore a little higher.
ps. asus board, the p5qc.