You're probably not going to be able to get much better framerates without updating your hardware. If your onboard card doesn't take up the AGP slot, you could go get a cheap geforce series card for like $40-$50.
The following settings may help some, however:
gl_dynamic 0
gl_texturemode gl_linear_mipmap_nearest
r_caustics 0
gl_anisotropy 0
Use "seta" before them if you want the settings to stay permanently.