The A7N8X boards ship with FSB jumper in the 1-2 position for
266/333/400 FSB mode, so you may need to change that to 2-3 so it
detects the CPU correctly in AUTO mode, or just set it up manually
(100 x 8 = 800 MHz).
There's a note in the manual saying processors with a core speed of less
than 1 Ghz should not be used, but it doesn't give any reason for this
nor is there any obvious one.