I am surprised that your vga card runs at all.
The GTS250 requires a 450 watt psu with 24 amps on the 12v lines.
Your current psu has nowhere near that capability. The card must be defaulting to 2d mode which requires less power.
I suggest a psu like the corsair 450VX or better:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003
Do not go by the wattage alone. Look for a unit from a quality vendor like Corsair, PC P&C, Seasonic, or Antec.
A psu is NOT the place to economize.