Wait until the next series of ATI cards and core i5 come out and decide. You can check out Toms Charts and look at CPU/GPU benches to see how you stack up to the current things. If you ask me you could see quite a good increase, but I would wait until DX11 cards come out (hopefully within a couple months). Core i7/i5 are quite a good CPU upgrade as well. You are 2 generations behind! (2&3 gens on the GPU once the next round comes)
Jumping up to a newer graphic card while waiting to hear about the likely impact of the i5 on the i7 pricing would seem a good start.
I don't see how your going to get much bang for your buck going the i5 route as you already have an overclocked Kenty and they are not much slower that i7 on most stuff you want to play anyway (argueably about the same).
Here is something for you to look at in terms of some better cards.
Its not your chip, its your operating system
you obviously did not hit a temperature barrier
this is my story
I have a Q6600 B3 with Corsair H50 cooling
I have it overclocked to 3.1ghz with load temps of 50C on each cores
In Vista I can't even overclock past 3.1ghz
I have Windows 7 on another drive, I overclock my processor to 3.4ghz and guess what it works
Why do you ask "will I get more frame rates" ? What you should be looking for is more fluid game play. The 8800gtx is still a decent card but there are more out there that will give better game play at high settings with increased eye candy enabled. I've seen a lot of people who say that turning on AA, for example, doesn't make a difference at higher resolutions but I disagree. The cleaner and crisper you can make things look the better the gaming experiences IMO. The 8800gtx will "tank" in some newer games when AA is applied.
As far as spending $800 on a newer system, I'd wait a year or two. the i7 really isn't a big jump in performance as we speak. I'm not going to do it for that reason alone and I'm a nut for the next best thing. Gains are going to be minimal.
DX11 cards will be out this fall and provide another jump in graphical performance with a new generation of cards. Personally if your able to play your games now, I'd wait a few months for a DX11 card.
Processor wise, the C2Q and Phenom II are nearly as powerful as I7 in games, you can look at the charts and see they're within a few fps on most games. My honest opinion is I7 is a waste of money for a gamer to purchase.
Aweiously your going to get a much better result sticking a new graphics card in and running the Q @ 3 Ghz or so ... you won't get any better frame rates running at 3.6 with the same graphics card ... unless it is at Wolvenstein resolution.
Get a GTX or a 4870X2 if IT suits the games you like ... the Steam stuff. Check the reviews we suggested to make your mind up in terms of value.
You will relish the increase in eye candy ... makes the games more enjoyeable too !!
A second GTX8800 doesn't bode well in SLI for some of the newer games ... it is a bit hit and miss.
Hope this helps ... if you had posted in the graphics section the guys there are uber at this sort of advice ... I am bit a pleb in this regard ... sorry.