Don't do anything right now. I cant believe you feel anything like slowdowns in games. Q6600@3.5ghz is quite fast and any other 775 cpu wont deliver enough gain so that you would notice a huge difference in games. Wait not for i5 and others but even for the generation of gpus and cpus that will come after 58xx and i5. Anyway, if youre rich and want not to feel difference but have the best that is on the market then feel free to upgrade to i7. :-)
The true question to ask is why you want to upgrade? What isn't your system doing now that you wish it could do performance wise. This anwser will better help determine if or what might require an update.
My CPU is quite dated and I feel it may be bottle necking my system a bit. Looking over charts and such, I can see gains of over 10% by switching out my cpu alone. I would like smoother/faster frames on the new games coming out and I have no plans of upgrading my video cards atm, so I figured my cpu would be a reasonable upgrade for now. On the other hand, I feel that I am at a dead end with my motherboard & memory and would like an upgrade path for the next few years. I basically don't want to be under equipped when new games start hitting the shelves next year. I wish I had a crystal ball and I could see what is coming down the pipe :-)
PS: I am running an E6600 that I have had since C2D was launched :-) I wish it was a Q6600
IMHO... Your best bet is to upgrade to the Q9550 and keep your current system. With your 4870's in Crossfire, I don't see you having issues in the near future once you add the Q9950. This time next year, reevaluate your system again to see if still meets your needs. At that point, a new system might be your best option or even just GPU updates.
I see the Q9550 being a good gaming CPU for a few more years. Just look how the E8000 series CPU's are still performing very well in gaming systems. They might not be the best for CAD work or whatever but are fine for everyday use & gaming.
Sorry, mad bad. You have c2d so dont swich to e8xxx, it doesnt boost performance that much. I've got firstname.lastname@example.org and my bro has e8500 and they are equal. Just like tecmo34 said, you can buy Q9550, but still, I see no real necessity.