My first suggestion is to overclock your CPU to at least 3.2 GHz and that should provide a better performance increase. Over 3.0 GHz is good for gaming performance.
Most of those games will benefit from a Quad-Core CPU but that looks out of your budget, so overclocking is your best option. Also, you can run the games at a lower resolution, which will help with the lag, as well
It is not the best of PSU Do you already have it or do you have options for a different brand?
I tried to Google it to find the 12v rail amps and had no luck (not a good thing... IMO). On paper it has the power for a GTS250 but I'm not sure in performance it does. It might just have to be one of those, let's see what happens.