First thing I'd try is removing all the PCI cards, then see if the system will power on.
If it comes up, try adding cards back one at a time & find the bad card.
If not, then it is either the power supply (Just because the MB LED is on, doesn't mean the P/S is good) or the motherboard.
If under warranty, call 1-800-624-9896 if not get a new power supply 1-800-372-3355 is Dell Spare parts
Inspiron E1505 /XPS 10 Windows 8
Dimension 8400 Windows XP/Vista/8