Check for a revision number on your motherboard. According to Asus: "Only PCB R2.0(or higher) support Intel® Core™2 processor" - yours might be earlier, so your new CPU will not work with this board if that is the case.
Also, does everything work if you put your 805 back in?
In short, you're screwed - a BIOS update won't help, you'll have to get a new mobo.
I don't recommend getting a Rev 2 of that board, since it's based on the old 945 chipset - which wasn't designed for the C2D.
Look for a board with the 965 or 975 chipset. If your looking for a "run of the mill" board (i.e. some overclocking potential, no SLI or Crossfire) - I'd go with either a plain Asus P5B or a Gigabyte 965-S3.