I’m running debian “unstable” and my PC have a - Intel core 2 quad 2.83 mhz with 4gb RAM, and with NVIDIA GTX 275 (896 mb). Why does Blender 2.5 lagging when for ex I modelling with subsurf on with level view 3? Same if a made a “big” lawn with about 50.000 “grass-blades”? Is that normal with a hardware I have?

Are you using NVidia proprietary drivers? If not try them.

Yes, I do…