panos theodorakis :
For what are u saying both I can't overclock my pc. I can't just overclock my gpu only I am going to have better results?
First of all, you can only overclock your GPU to a point. At best you MIGHT be able to get the performance up to the next level up on the GPU... but that would be optimistic. Another thing about overclocking is that it is based on percentages. Lets say a high end r9 GPU is rated at a 5, and an r7 GPU is rated at a 2, if you did a 10% overclock then it would bumpt the r9 to a rating of 5.5, while the performance of the r7 would only be raised to 2.2. Those numbers are pulled out of my a$$, but you get the idea: when you start with less then overclocking makes less of a difference.
The other issue here is that not only will your i3 processor not be able to be overclocked, but it is also going to be a limiting factor in many newer games. Even if you bought a big monster GPU like an nVidia Titan, you are going to have a hard time playing newer games at high levels simply because your CPU cannot handle the workload.
All that said, don't let that discourage you. Everyone needs to start somewhere, and this will certainly be a large upgrade for your computer which will be very noticeable. Start with a decent PSU, then get as big of a GPU as you can afford. Then save your pennies and get yourself a case, motherboard, and gaming processor, and move your ram, HDD, PSU, and GPU to your new rig.
Most of us gamers do not start out with high end systems right away. You typically start with a basic computer (like you have currently), and find an upgrade cycle that fits your budget/needs. I personally have 3 cycles going on in my PC at home. The GPU is replaced every 3 generations (3-4 years, can't wait for the GTX800 series later this year!), the core system (CPU, mobo, RAM, and HDDs) is replaced every 4-6 years (waiting for skylake/skymont in a few years), and then other components like the case, PSU, keys, mice, and monitor are replaced typically every 7-10 years (as needed). Staggering upgrades like this allows you to grow your system in a way that you are never too far out of date, and so that you never break the bank replacing everything all at once.
Also, put the word out there that you are getting into computers. You will be amazed at what people are getting rid of and are more than willing to give you for free or cheap. Take those parts, put together working computers, and sell them for a small profit. Use the knowledge and money from those efforts to fund your gaming addiction. Doing that you will have a decent game rig in no time, you just need to start somewhere.