If you already have a Q6600, you probably shouldn't upgrade to any of the 9x50 series quads. None of them, outside of the extreme, will overclock better than the 6600. Aside from that, the only difference will be slightly better power efficiency, and the larger cache. IMO, if you already have a quad, save for the Nehalem platform.
On the other hand... If you're like me and built a temporary dual-core solution around the eagerly awaited 9450 and 9550, then these might be worth it.