CT-7NJL3 moved from XP 2100+ to XP 2700+ now just wont boot

When this has the 2100 its rock solid, on all day. I put the 2700 in
and its never booted at all at that speed, can get it to boot at 1500
but when I "load optimised" or basically do anything with the FSB
settings it just won't boot, I get power but no beep as the m/b boots
and its hung. The power stays up, I mention this because if it was the
cpu fan I know it would shut down. PSU is 300W. Totally repeatable. Is
the combo compatible? If its a dodgy CPU I can return it - does this
seem the cause? Thanks...

One other thing - I have another Chaintech which came with a Windows
Overclocking utility on the CD. This board I'm having the eproblem
with is NForce2 based and theres nothing like that on either the CD or
the CT site. What do you use?