My computer is a few years old and plays online games pretty well, but the onboard graphics aren't very good.
I have an HP Pavilion p7-1110, running windows 7, an i3 processor, 1 terabyte hard drive, and 6GB of ram. My power supply is only 250. Which graphics card should I pick up for it? (preferably Nvidia)
Your options are going to be extremely limited with that power supply. The GT640 is about as high as you'll be able to go. I would stay away from overclocking it on that PSU as well. Still, it'll be far more preferable than onboard graphics.