I'll say the q6600 hands down. Despite the 6850's high stock speed, "only" 2 cores is its downfall. Even if these days multi threaded apps are sparse and even if I dont get any more frames in Crysis with a$280 Q6600 than my $115 e4300, things wont always be like that. Besides, the Q can reach 3Ghz and beyond quite easily.
For those willing to spend big on their CPU's q66 FTW. Otherwise, wait for the 45nm derivative of the q6600 when its available.