I have P5K Deluxe with Quad 6600 in my new rig, back in shop they told me it's the best choice for Q6600 today. I rarely believe that shop-talk, so I double checked this in different techie forums. Now I am very happy with it, just like I was happy with my veteran Asus P4C800 Deluxe.
As to P5W models I wouldn't recommend them, they are confirmed to work with Q6600, but they're old models (heck, >1 year is old in comp world) and price is about the same as that of P5K.
As to Q6600, try to buy newer model, with G0 stepping (less heating). It should have this sequence of letters at the end of serial number: SLACR