1.5v is the max stated voltage that CPU can take. Because of this, I would say the guy you've seen running at 4.8GHz is either on some amazing cooling, has a £400 motherboard, a 'freak' chip that can take that voltage, or is simply burning his CPU and it will one day fail (maybe a mixture of all 4?).
If you increase your voltage to 1.5v I would think you could probably get more from your CPU. Maybe 4.3GHz? Just a little at a time. Maybe increase your FSB to 201 and see if you can boot and run Prime95, then 202. etc etc. As soon as you blue screen up your voltage by the smallest increment you can and try to boot.
My suggestion would not to go over 1.5v as your computer doesn't run very fast once your CPU turns to liquid .