I built my PC about 3 years ago:
Phenom x3 720 2.8GHz (OC'ed to 3.2GHz)
4GB RAM
Radeon HD 4850
It's served me well, and for the most part I can run Skyrim on High settings. However, there are a few areas that are extremely laggy (low-ish FPS and a hell of a lot of mouse lag). These aren't CPU intensive areas, but in a few forests, and areas with lots of fog particles (such as marshes, snow topped mountains), and so I'm looking for a GPU upgrade.
My budget is about £150, tops. I'm currently playing on High-ish settings at 30-35fps. Preferably I'd like to play at Ultra with 40+ fps, if possible.
If I could have any recommendations, that would be appreciated. I kind of tuned out of the world of GPUs after I built my PC so I'm not really informed anymore.
Cheers guys!