Haha I wish my ram can run at those speed but I do have a divider 5 Ram/6 FSB and it's actually running at 208 MHz. At 2-3-2-5-1T my ram can only do 220MHz stable. As for the increase to 255x12 I've tried it and it's a no go cause going 251MHz I was already getting errors in prime.
Your chipset shouldn't be holding you back but just to make sure, I would increase the voltage from 1.6 to 1.7. Have you isolated your chipset to find it's limits?