Ok, try re-seating the memory. Power down, unplug power, remove ram, power up, see if it posts. If it does you have some bad ram and you need to figure out if 1 or all of the sticks are bad. If it doesn't, put the ram back in and look elsewhere (could be processor or MB). Power supplies seem to be working no?
If this doesn't work, if you're feeling adventurous, you can try to re-seat the heatsink(/s) on the processors.
Also a dumb question, but is the video cable secured properly?
Also, is the fan louder than normal? This is a sign that the motherboard is not controlling the speed of the fan and could indicate a bad motherboard. Do you still have the HP Carepack with the G3, or was this purchase not from HP?