Seriously, is there no one here who has any idea on what to do next to diagnose this problem?

You have a hardware problem somewhere unfortunately the diagnostic light error codes are of no help.  The best I can suggest is try the trouble-shooting tips I have listed below.  If this problem started after the graphics card was replaced then I would reseating all hardware and cables first as you may have bumped something loose while working inside.

 

Try clearing the NVRAM (CMOS) to see if it resolves the boot problem.  Possibly the information stored in the NVRAM (CMOS) became corrupted which is not uncommon.  Normally you would clear the NVRAM (CMOS) by entering the BIOS Setup, but it would appear that you can not do so.  The alternative would be to remove the battery from the motherboard to clear the NVRAM (CMOS).  With the machine unplugged from the wall remove the battery from the motherboard.  Then with the machine still unplugged press the On button for 30 seconds to dissipate any remaining electrical charge on the motherboard.  Then re-install the battery, plug the machine back into the wall and see if it will boot normally. Depending on the age of the system you might try installing a new battery when clearing the NVRAM.  A failed battery can cause a variety of boot up problems. A new 3-Volt, CR2032, coin cell battery can be purchased for around $3 (US) and is available at any Radio Shack, computer stores and most discount stores like Wal-Mart.

If the machine still fails to boot then strip the motherboard down to the bare minimum.  By bare minimum I mean remove all expansion cards, disconnect all drives and leave just the processor, memory and graphics card in the machine after reseating them.  If the computer boots this way then start connecting each device one at a time until the no boot scenario returns.  This would then indicate which device has failed and is causing the problem.  Also try booting the system without the keyboard, mouse or any other device plugged into the rear of the system.  A faulty USB device has been known to cause problems. If you have more than one memory module (pair with RDRAM) installed then try booting with only one and then swap them around and try again. If you only have one memory module (pair with RDRAM) then try booting without any memory modules installed. Also try swapping the graphics card for another one.  If this procedure doesn't resolve the problem then you maybe looking at a situation where the motherboard or processor has failed.

I really appreciate your response.