Buying GPU will CPU be bottleneck

A Q6600 is still a pretty decent chip. At stock it might limit an HD5850/70 some depending on what game is involved but not enough that they should be avoided imo.
Also that CPU OCs easily and well. If you OC I wouldn't worry about a bottleneck at all.

I tried OC but too complicated to learn so im on standard  

How can i know when a CPU will bottleneck a game ? What things should i look for to know which games are going to be affected in future.

Basically if you increase the resolution and the frames per second don't decrease much or at all then there is a CPU bottle neck.
Like I said I wouldn't worry about it too much, the processor is still pretty decent. There will always be a bottleneck, otherwise your frames per second would be infinite. The question is just whether it will be the card or the CPU.
I would recommend giving overclocking another try however. A lot of guides can be exceedingly complex filled with info you don't really need to know or think about. Basically you are just increasing the front side bus(FSB) in the BIOS which increases the speed of the processor. If after doing so the system is unstable then you either need to scale it back or bump up the voltage slightly. It's almost impossible to damage your system OCing these days as the processors have built in temperature sensors and shut themselves off automatically before temperatures can get truly dangerous. Just look up what a decent OC for your processor is and then try for half of that at first and if that proves stable bump up the FSB in smaller increments and the voltage as necessary until you are happy or your temps start getting above what is considered wise. I believe 3.25ghz should be pretty easy to achieve on that processor even with just the stock cooler.

Symptoms of a CPU bottleneck would be if you ran a game at several screen resolutions and your FPS is the same/similar for all of them.

http://www.tomshardware.co.uk/radeon-4650-agp,review-31...

Take a look at the chart on this page, notice that the FPS is the same for all three resolutions and also notice that the other cards of similar power are also limited to the same FPS.

Yes. It's a disease.  

anyways that it all depends on what GPU you buy, although it is a pretty good CPU indeed. Just make sure you shove a nice cooler onto that thing before you OC. it's pretty safe helped me get over bottleneck of part of my GTS 250

At 2.4 ghz your CPU will not drive a higher end card anywhere near its full potential. Nowhere near.

Should that limit your choices on your new GPU? Not at all. Buy the best GPU you can possibly afford. Then you have the comfort in knowing that when you decide to upgrade your CPU, your GPU is going to scale nicely upwards with your CPU speed.
A gaming PC is always a work in progress. When you upgrade, always buy the best you possibly can for your budget. Never under buy because you are afraid another area of your system will be a "bottleneck". Unless of course the entire platform is too far behind the curve, and not worth spending money on at all.
The theory of constraints (or bottleneck) used in production flow can be applied to PC building as well I believe, with a little side thinking.

1. Identify the constraint (the resource or policy that prevents the organization from obtaining more of the goal) -The goal here is smoother faster, better gameplay.

2. Decide how to exploit the constraint (make sure the constraint's time is not wasted doing things that it should not do) -The current hardware is optimized and working perfectly, it ain't going to get any better.

3. Subordinate all other processes to above decision (align the whole system or organization to support the decision made above) -This translated roughly could mean save your money and research for new hardware, pinpoint the exact thing or things you want to upgrade.

4. Elevate the constraint (if required or possible, permanently increase capacity of the constraint; "buy more") -Buy the best hardware you can afford.

5. If, as a result of these steps, the constraint has moved, return to Step 1. Don't let inertia become the constraint. -Now the bottleneck will move to another part of the machine. So you go back to #1 and start all over, as your budget allows.