For me, the motherboard is a FUJITSU SIEMENS Model XY680 0.3B (CPU-Z info) ; I do not know if 0.3B makes a difference
The P8400 was installed in my computer and I just installed the P8800 and it works very fine.
The processor performance index of windows Seven64 was 5.8 or 5.9 with the P8400, it is 6.3
with the P8800.
P8400 is a 25w proc, 2.26 ghz, 3MB cache L2, coef speed 3 and 8.5
P8800 is a 25w proc, 2.66 ghz, 3MB cache L2, coef speed 3 and 10
Look at my french post of this upgrade: http://forum.ts.fujitsu.com/forum/viewtopic.php?f=110&t=45847