Gaming cpu lga775

What is your max budget and motherboard model? Just because your board has a socket that is the same as a specific model of CPU, does not mean that it also supports THAT specific CPU, so it is necessary to check the supported CPU models on the boards specifications.

 
The 915 and 925 are both S775, but neither chipset supports the C2D or C2Q series. The 945 at first didn't, but then did support them, so the revision of the board matters greatly. If you have a 965 or latter you should be fine. Nvidia made chipsets for these, but they don't support the 65nm C2Qs. They will run the 65nm C2Ds ok, but not the quads. Q6600 will be fine on these, but not the Q9xxx series.

As for the "best", that would be the QX9775.

https://ark.intel.com/products/34692/Intel-Core2-Extreme-Processor-QX9775-12M-Cache-3_20-GHz-1600-MHz-FSB

Super rare and you aren't likely to ever see one. The QX9650 was much more common, but even now they are often though not always sold at a "halo" price. The Q8400 and Q8200 are easy to find and are usually very cheap. If you can find a 9650 for cheap then go for it, otherwise I'd get the 8400/8200 and call it a day. I wouldn't spend more than $20 on tech this old. If you are using a S775 system you really need an upgrade. Even a $300 machine from your local big box store will be many times faster then these old CPUs.
 
YOU said it can, but can it? As darkbreeze said, what board is it? Again, if it's a 915 or 925 chipset board, it CAN'T support a C2Q. You voted down my answer and you don't even know what a C2Q is? It's a Core 2 Quad, or a quad core CPU. But just telling us it's a S775 DOESN'T mean it can support a quad core. 925 board? No support. First gen 945 board? No support. Nforce590? Socket 775, but no 65nm quad core support. Just because it's a S775 DOES NOT mean it can support a quad core CPU. You can vote my posts down all you want, but it's not going to change that simple fact.

I even finished my post by listing chips that MIGHT run in your board. Instead of being mad at me, why don't you go look for one.