The first thing you need to do is replace that 250 watt PSU. You can't run the graphics card you listed with that PSU at all.
Also, what resolution is your monitor?
I recommend the following PSU:
Corsair CMPSU-550VX 550W - $79.99 after rebate ($89.99 before rebate)http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
As you can see it's a very popular PSU because It's sold out at the moment. They'll have more in a few days.