Ramerix,
I got this mobo working even with 500fsb (posted successfully, with increasing the NB voltage)
My opinion is that these mobos (DX38 and DX48) are sensitive to some frequencies!
Try to disable "Watch Dog" in BIOS, because it shows the unsuccessful messages and prevents you from gaining more FSB speeds.
At least doing this, worked for me, hope to work for you too
And if you don't know, first change the Mem/FSB ratio to 800/400 and decrease your CPU multiplier to lowest value, before trying to find the highest working FSB settings

Good Luck