i have been run an 8400 with both 0504 and 0404 bios (now with 0404). they are both very stable.
the advantage fo 0504 is it gives you .5 multiplier, but on the other hand, i found it overclocking memory much worse than 0404, and 0404 gets higher scores in memory bandwith test in everest..i can run 4 sticks of 1G d9 at 1090 5-5-15 with 2.10v with 0404, 8 hours prime and memtest stable. but couldnt even boot beyond 950 5-5-5 with 2.10v with 0504.