You do NOT need a 350 W PSU in order to run a 6600. Looking at
this article at XBitsLabs reports that a 6600
GT draws less than 48W and a 6600 GDDR2 gets by with ~28W.
Since the 850 MHz Pentium III in the above mentioned HP uses less than 30W I fail to see what would be consuming the other ~275W. The built-in toaster-oven?
/Tiny Overkill