Hmmm well if my maths is right with a stock FSB of 266 for those processors it must have a 10x multiplier to be 2.66Ghz (stock). So that in mind you should be able to set your multiplier to 10 and fsb to 400 and get 4Ghz, with your RAM on the lowest divider it will run at 800Mhz, which is stock for it. But getting that sort of speed will need a lot of voltage to the CPU and air cooling probably wont cut it. 3.6Ghz is probably a more realistic target, (but give it a go and see what happens it might be fine, ive not got a dual core that high myself).
Im not sure why your multiplier is 6-8, possibly its set wrong the bios and also speedstep may be running the bios as well which is dropping your multiplier to save power. I would recommend turning that off.
So in summary I dont think you need a new mobo
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Hawker