I have 1 Gb of Ram, I guess that's why when I uninstall it, I feel I gained a lot of performance. Also, because I formated my PC recently, and I noticed the before and after Catalyst. May be if I would have had more junk on it, Catalyst wouldn't have make big difference.
With 1GB of RAM You should use WindowsXP and ATI drivers from same year the video card were released. I guess from year 2006 or 2007 will work fine with no performance hit.