How much can I OC my CPU?

So, as far as I understood, you want to OC the AMD CPU? If so, it all depends on your cooling system. With a decent air cooler you can get up to 4.0 GHz (all depends on the CPU itself) or up to around 4.3 with water cooling. While with the stock cooler, you can barely get anywhere, it already has a hard time cooling it at stock settings. Also, I'd recommend you to wait a bit, AMD is going to release Zen pretty soon (end of year) and also they will introduce some new GPUs.

Some questions and comments.

What is your motherboard? The power delivery system on your motherboard may limit and overclock.
Do you have only the stock cooler? An good aftermarket cooler will be essential for a decent overclock.

I must advise you to use another PSU. A good PSU can last for years and through many builds. It is a 'forever' part. It is vital to have a high performance PSU for and overclocked/gaming system to ensure long and reliable operation. Are you in Brazil? That model is available in only some markets.

Unfortunately, there are no choices I can recommend in that price range.

I would go with the system you have and overclock it as much as possible in the search of more performance. When you get an new CPU, you will need a new motherboard, and may need new memory too, so start saving for a major re-build.

If you need to change CPUs in the next couple of months, Intel is the only good option, otherwise wait for the next AMD generation and see how well it does.

As far as overclocking goes, you can adjust a few things to make your CPU faster.
CPU frequency is: base clock (BCLK) * CPU ratio
Base clock is usually 200 MHz to start.
CPU ratio sets how much faster the CPU runs.

With that CPU, your CPU ratio (or clock multiplier) is unlocked. Which means you can set it higher than it was set from the factory.
By changing the base clock you will effect the memory speed, CPU speed, and hypertransport bus speed (CPU to northbridge speed).

IIRC, a good setup was < 4.0 GHz on air, and ~4.2 GHz on water.

Rules:
1. More speed or more voltage = more heat out, more power in.(Watch your temperatures carefully)
2. Small steps, the difference between nice overclock and unstable is a pretty narrow line.
3. To gain stability while overclocking, you may need to increase the voltage to the CPU, 1.60V is about as high as I would go with your CPU. (Theoretically)
4. If you go to far, you can reset/clear your CMOS and try again.

All of this depends on your motherboard and how old your chip is. The x4 965 is a very old chip and had issues going past 4.0Ghz on 64 bit operating systems when it first came out.