Remove everything that is not necessary - remove drives, all expansion cards (including the RAID controller), all but one stick of memory in the first slot, and optical/floppy drives. Try a single processor in slot 1, swapping if it still doesn't work. Clearing the NVRAM can be done via the jumpers on the motherboard - a better alternative to removing the battery.
Also, be sure to reseat the expansion riser cards as well.