Q8200 is slightly faster at stock speeds and slightly faster clock per clock than the Q6600. It's 45nm so it'll run cooler too.
But the Q6600 is a far better overclocker and when overclocked will soundly beat the Q8200.
But there is really no reason to go from a Core2Quad to another Core2Quad. Get the Q6600 if you can return the Q8200 for a refund. Otherwise, stick with what you have currently.