You are really going to need to upgrade your PSU as well. Can you tell us more? What OS do you have, is this a branded PC or built machine (for PSU concerns), What games do you play, what resolution is your display, etc?
I was planning on getting a better PSU. I have Windows 7. My PC is branded. I play Garrys Mod and Empire Earth. I want a graphics card that could handle Garrys Mod because i have to put my graphics, textures and ect to low settings. My resolution is 1280 by 1024 pixels. I think.