What video card should I get to play BF3?

I currently have an ATI Radeon 5450 which runs Battlefield 3 only at lowest settings and lowest resolution. I want to play it much higher without 12 fps. I have around £100 to spend on a video card. My CPU is an AMD Athlon X2 250 3.0ghz and I have 3GB of RAM. Which GPU should I get?|||100 pounds will get you a HD 6770 maybe a 6790 or Nvidia 460 or 550Ti either one will give you a significant boost in fps compared to a HD 5450 on low to medium settings but there is a draw back,you will have to have a powersupply of 400 to 500 watts if you dont have one then you will need to increase your budget and get a new powersupply as well.