i have 3 quadcores.
2 are xeons, 1 is a Q
X3220, X3210, Q6600.
If your talkinga bout those xeons, then yes you can overclock them, however i highly recomend the gigabyte P35 or the Asus P35 boards. They handle quads very very nicely.
Now if your talking about overclocking a cloverdale, you need to pin mod it. I suggest you go on Xtremesystems.org/forums And pm a pal of mine named dave. His nick over there is Movieman.
If you have a cloverdale overclocking question, he's probably the best person in my book to consult.