Hi All,
FYI, I've "upgraded" my iMac 7.1 (Mid 2007 imac 24' 2.4ghz - T7700 intel chip) to an X9000 chip.
Result : iMac working and fully functional but slower than with the original T7700 chip (geekbench results : 1850 with X9000 and 3350 with T7700). Looks like the chip indeed is working at 400ghz, and that it's not only an "about this mac" report bug.
I tend to think that the Apple EFI / BIOS does not properly recognize the chip because it's never been written for it (The X9000 chip has never been officially listed in the iMac range).
=> back to original configuration....
Sorry guys...