Thanks Piru,
I did try again with the changes you mentioned and I got some better results but couldn't get as much as 140fps in 800x600 someone mentioned. Is that max fps or average and at what settings? Best I could get in 800x600 is 124,6fps with all settings at lowest (16 bit, lowest textures, low geometry, vertex lighting, bilinear filtering). On highest settings (32 bit, highest textures, high geometry, lightmap, trilinear filtering) in 800x600 I have 60,0fps. That's not bad but If you got better results on Mac Mini I'd like to know how...
My Mini's CPU is 1,83GHz G4 (overclocked 1,5GHz model) so I guess the CPU is not problem... The rest of the configuration: 64MB VRAM, 1GB RAM, 160GB HDD.