I switched from a E6600 @ 3Ghz (OC) to a Q9550 @ 2,83 Ghz (stock) and it was a giant slap on my face .... then I OC'ed up to 4 Ghz (471 x 8,5 on a Gigabyte EP45-UD3P ) and It was the same huge slap again ...
then I builded a Raid0 array and again .. another f...g slap on my face .. it hurt .. but this is gooood

Edit : acording to this, you can expect 3,4 Ghz @ 400 Mhz FSB with the Q9400 ...