What do you have now? Have you looked the the Performance Charts posted here on Tom's to compare the 2 GPUs in question? Also, what make/model of power supply are you using? Last question, what games do you plan on playing?
This might prove useful:http://www.tomshardware.com/charts/gaming-graphics-cards-charts-2009-high-quality/compare,1831.html?prod=on&prod=on
Good luck!!! BTW, looks like the 250 is the slightly stronger of the 2.