Will you OC the CPU? Games akin to FSX and GTA4 will play improved on quad evaluate to a dual. They're more CPU dependant than GPU.

If you do some video/image editing you're enhanced with a quad. More games will begin to employ all 4 cores.

So to answer your question I believe that you're enhanced with the Q9400 as it should last you longer and be able to remain you computer longer. I'm thinking to switch my E8400 with my Q6600 (OC to 3.0 GHz) for my gaming rig.

If I amend my processor I will not OC the processor, the Q9400 is only 2.66 GHz and I observed on some benchmarks that the E8500 is enhanced from some fps more. But I recognize that quad cores are also good option for future. And I don’t recognize if I will have to change of in use system because now I have an E4600 and if I choose the Q9400 do you recognize if I have to modify of operating system?

My E8400 is store currently but I OC it once in a while to 3.6 GHz, it's huge now for games but I believe than my Q6600 OC to 3.0Ghz will provide me enhanced gameplay, I currently play games akin to Ovlivion, FSX, Crysis, AOE3, civ4 ...

I should observe an augment with FSX but I don't anticipate much for the other games.

I'm starting to observe the limits for my Dual Core CPU with some of my video/photo editing software.

Well as I cannot overclock with my HP product: S. I believe that I should stay that the Q9550 cost goes down because Q9400 appears not having enough GHZ and cache or only purchase a E8500 because I do not actually edit videos or multitasking...I only play most recent games.

A Q9400 is better than a Q6600, 2.66 vs. 2.4. If you play only the most recent games then in some months when you'll obtain a quad optimize game and observe the bench mark and observe that yet a Q9400 obtain more fps than a e8500.

Now the E8500 seems good and better than a Q9400 but what will you believe in 6 months from now?

FSX and GTA4 are quad optimize and you'll observe a nice augment in fps, not a lot but maybe sufficient to make a difference.

But if you can anticipate the Q9550, it'll enhance. You should imagine about how long you'll want to stay this pc because if you'll modify your pc in 1 year then it doesn't actually matter what CPU you obtain now. But if you want to stay your pc as long as probable then wait for the Q9550 or obtain the Q9400.