Speaking from the perspective of an E8400 owner I do not think you will see much more performance than you already have, until AHIII. Once you are in to AHIII the GPU will be more important, but as of now I believe your CPU is holding you back.
Since you have probably been using that same system for a number of years I would not update your video card until you update the entire system. Dell PSUs are notorious for providing just enough power to sustain the system as it was originally built. Of course, if you are running something like a 4## series you will be using less power, but . . .