I've just done a new test... 672 mhz cas 4-4-4-4 and 3-4-4-4... i gain about 0,4 sec betwen cas 4 and cas 3... after 2 pi the system frozen down... (thanks AliG)

and just 10 minute ago I tryed to boot over 680 mhz...but I need more vram...windows frozen,,,