Btw why do you have a great CPU but a crappy PSU and a crappy GPU? Makes no since.
Different computer tasks require different types of hardware...some tasks, say for example Super Pi or other mathematical based programs, are incredibly CPU/memory intensive yet require absolutely no GPU power...I assume this is a general purpose web browsing/word processing machine that has been built with the intention of being prepared for multiple threads in the future
As for the issue at hand...your BIOS may indeed be outdated but in this case it would still boot, but only identify the single core and not the second core on the die
Another possibility is the fact that the BIOS is so dated that it cannot recognise the newer Athlon revisions using 90nm process sizes
Diagnosing your problem...pull everything out of your PC and assemble it all out of a workbench...take off all non-neccesary components such as hard drives and periphiral cards and boot...if the problem persists continue removing parts until you're simply left with the CPU in the board...the object of this is to get some sort of beep code error that way you can identify at what stage of the POST the issue is arising and moving towards correcting it
Make sure your speaker jumper is set in the ON position so you can hear such beep codes and make sure you have the beep codes manual for your bios handy