The only problem i had with the 0604 (i think thats what ships with the p5b-d now) bios was double reboots, but that was because i overclocked it.
If you plan to oc your cpu, then most diffidently update it. But i would update it any ways, i havn't had any problems with the 0804 update and it fixed my double reboot problem which was driving me crazy
It's really easy too. Just get a jump drive and dl the 0804 (should be the lastest bios) bios file from the asus website and enter the bios setup screen (hit del when booting) and go to the tools tab and enter the "ez utility blah blah" (sry i don't remember it exactly
) and find your bios file on the jump drive and update it.