Upgrade CPU - cheap w/SLI or expensive wo/SLI?

I am looking to take some of my tax refund and dump it into my computer.

My current specs:
EVGA nForce 680i SLI MB
E6750 – overclocked to 3.6
8800 GTX – overclocked (I don’t remember how much)
4 Gigs Ram
1 kw PSU

I am looking at a couple different possibilities involving either a chip upgrade a video upgrade or both. All I do is game with this PC. Currently, I believe (If I am wrong let me know), my CPU is the weakest link in my system so I want to focus there.

Should I buy

A Q6600 ($189.99) – and overclock it and another 8800 GTX ($179.00) for SLI (total ~$370)

Or

A Q9550/9650 (~$300) and skip the 2nd video card. (Total ~$300)

Or

Go all in and buy the better chip and the 2nd card. (Total ~$475)

Obviously the best option (if money were irrelevant) would be to buy the best chip and the 2nd card (actually if money were irrelevant I would go I7 and get a new video card). But money is a factor. What I am looking for is the best bang for my buck.

Also what is the difference between the 9550 and 9650 beside the freq.? Does one overclock better? Does one run cooler?

There is nothing wrong with my system. This upgrade would basically fall into the "because I can" category.

All games I play I run at 1680 x 1050 (My native monitor res.) Most I run at very high settings. Some I can crank up the AA some not. The game that has been taxing my system the most recently is GTA 4. That is where the inspiration for the quad core came from. As far as I understand GTA will actually utilize the 4 cores.

An E6750 @ 3.6Ghz is all you will need for a long time... Just keep it and upgrade the GPU as much as you can.

I agree, the graphics card is your most important upgrade. I think since you already have the SLI board, adding another card is a great option, unless you can sell it for a good price on ebay and then get the GTX 285. You can SLI that later, mr. 1K power supply, haha. I really think either graphics upgrade is a good one, it boils down to finances. If extra money is not that important, GTX 285 is the future, its gonna be better in the long run.

Q6600 if GTA4 is really gonna improve that much with it on top of the graphics upgrade. Don't dump your board out for an i7 setup, its not really going to be worth it. Wait for the next gen.