looks quite well so far ... i don't see a reason why it wouldn't work reliably with 16 GB
a few comments though:
double check power consumption of all components under full load and consider you might add another harddisk later ...
my feeling says you should be fine with the 500 Watt ....
trade the digitus gigabit NIC for a IntelPro 1000 PCIe or stay with the onboard one
the Q6600 is 2.4 GHz AFAIK and has FSB 1066 (themal design power 95 W)
so consider using also 1066 DDR-2 memory
strictly stay with memory specified to be compatible by the motherboard vendor
or visit kingston's website to see their compatibility list (RAM with motherboard)
christian
ps: we didn't upgrade our P5x board to 16 GB so far and intel boards are also my favourites currently, but finally all of them are made at the same factory in taiwan ....
and remember: only a CRAY can run an endless loop in just three seconds.