what are u talking about kid u better know ur facts before talking too! He's got a core2quad @ 2.7Ghz THERE IS NO NEED TO UPGRADE CPU immediately...people can play CSGO just fine even with Intel i3's so stop saying things without proper knowledge!
Get a more efficient GPU instead w1thout, something like GTX650 its quite good and not expensive as much ;)
You can't just upgrade ONE component.
A cpu fits into a cpu socket which differs with each type of motherboard. For example: an i5 3470 will need a 1155 socket which you can find in H77,Z77 and B75 motherboards. These motherboards also support DDR3 RAM, not DDR2 RAM (which is what you've got at the moment).
If you're going to overclock, simply get a 3570K. GIGABYTE GA-Z77X-UD3H is a REALLY good motherboard for overclocking, others also recommend the ASRock Z77 Extreme4. I'm afraid these components WILL stretch your budget a bit though.
Your old motherboard has a LGA775 socket, the newer generation i-series need an LGA1555 motherboard.
If you do not like Intel anymore, you could always go AMD, it might be a bit better for your budget.
It's not worth it to upgrade to another s775 CPU, if you want to upgrade your cpu you will have to get a new motherboard and probably new DDR3 ram too as your DDR3 is likely older stuff that takes more voltage to run to spec.
That GT520 is a very low end card and that is where you would see the largest gain.
If you could replace that card and give your cpu an overclock you should see a much more stable framerate.
Before looking into overclocking you should check what motherboard you are currently using and make sure it's capable. Do a lot of reading on the subject and you may want to pick up a decent cheap aftermarket cpu cooler.
The plus of getting a new video card and cooler is you can always move them over to a new cpu/motherboard/ram combo if you decide to upgrade more down the line.
You're actually right about the GT520, I even had troubles finding the correct specifications of it on google, and that means a lot.
But still, if he overclocks his CPU, he won't be getting past the 110fps in 5on5s mark on that CPU, no matter how much he overclocks it. For me it's really hard to decide whether he should upgrade his GPU or CPU, he still won't enjoy playing CS:GO.
Sure the maximum FPS number is important but so is the minimum.
With the system and budget he appears to have upgrading to a new cpu/motherboard may be out of his reach but even a budget lga1155 cpu and board would still be an improvement on his C2Q lga775 setup.
The problem with that is it would still leave him with a slow GPU, and although this game loves a fast cpu you still need to have enough GPU power to push the amount of frames needed for a smooth experience.
It may be possible to pick up a cheap 1155 setup and add in a cheap, used video card for the money he has to spend.
I know here in Canada I can pick up used cards such as the 5850/6850 for $50-80 but I am not sure on the used market where he is located.
Windows Vista @ 64Bit
E5300 Dual-Core @ 2.6GHz
NVIDIA GeForce 9600GS
In 5 on 5 matches, I'm at a constant 128fps (capped). In deathmatch scenarios, I'm shifting between 80 and 120fps (4:3r; 1024x768p; lowest graphic settings).
I overclocked my CPU to 3.2GHz, defragmented and cleaned up my hard drive, updated my drivers as well as the BIOS, used the launch option "-high" in order for windows to run csgo.exe at a higher priority, forced the refresh rate, disabled unnecessary processes and used the Vista AERO skin @ 0% transparency in order to safe computing capacity, set windows to prefer using the CPU to run games instead of the GPU and forced my GPU to let the CPU prerender the maximum of frames (you have to be careful not to suffer from input lag of your peripherals IN-GAME with this though). You also want to remove the dust from your PC's fans regularly and even add additional fans (especially if you OC your CPU) as this helps your system's stability and allows for it to go all out on game performance.
Buy yourself a new graphic card and OC your CPU to keep up with it would be my advice.