Even if you don't need 64 bit, the Athlon64 is still the fastest 32 bit processor, especially in SW. If you do upgrade don't forget that you'll need new memory and probably a new power supply. I've lost track of AthlonXP motherboards, but I know that new designs and chipsets are still trickling out.
"SYSTEM FAILED CPU TEST" sounds like something happened to the CPU itself, not the mobo. If you can, put the CPU into a compatible machine and see if that boots. (Don't risk a known good CPU in a potentially faulty mobo, if you can avoid it.) Since you're stalled without your machine, you might chuck it in the car and take it to your local geek shop. Someplace with a good selection and repair capabilities. Regardless of what's wrong, they should have what you need in stock, even though it may cost a bit more to get it and the help troubleshooting.
Even if your certain of what's wrong, a local shop could get you up and running today instead of tomorrow.
All this writing, and I still can't answer your question directly.