Hiyas,
The frame rates you are getting with the system you listed seem to me to be about right. I would recommend the 7.78 or 12.41 drivers for you ... and if you want to increase your performance, your CPU would be a wise thing to upgrade. Just to give you an idea ... I currently have a 1.2 Athlon with a GF2 Ultra card ... and I never drop below 35, usually around 40-50. In smoke sometimes I will dip down to 25 or so. This is in 32bit color, mip mapping, 1280X1024 res.
One thing you might check is the refresh rate on your monitor. If your monitor will support a higher refresh rate, that might help increase your fps.
Hope this helps.
Nim