It is advisable to swap out the PSU if the PSU is more than 3 years old.
It is not worth the risk. PSU over the years will degrade. One fine day when the PSU reaches its lifespan, it will just explode. And when it does, if the PSU is not good it will just bring down your valuable components together with it. Causing more RMA shyt for you.
Usually during an upgrade, PSU and GPU will need to be pair together.
On the other hand, you have a C2Q cpu which is quite old. Putting in a high end gpus MAY lead to bottleneck. They wont yield better results than a current cpu.
But since you have mentioned that you would wanna use it for casual gaming, do list what type of games you play? If it is for DOTA 2 then there is no need for you to change.