I am a big advocate of quality PSUs, but in this case I have to agree with Zhyr. It is bad, but certainly nowhere near as bad as some noname atrocities we get to see on a regular basis. Low power components as well. Keep the rig as it is now, no point in upgrading anything since it is balanced. You either upgrade everything or nothing in such cases.
CS:GO is not heavy at all.
Your Q6600 rig should be able to run it.
The problem is however, if you are aiming for 300 fps or so...like the other CS:GO freaks, you will not be able to reach such fps.
Do not upgrade your Q6600 PC, safe your money and plan to buy/build a new rig!