Don't know too much about it but did try it once some time ago. I found that if I pushed it too far the PC would just shut itself down with no permanent effects.
So it might be worth trying to push it a little further - but I'd say you're doing quite well with 1.4 from a 1.2 anyway (if it's reliable).
I'd recommend searching the web to find out what people have achieved with your specific processor (may have to take a look at it to determine the exact model).