thats because you have a geforce2mx. its already met its performance capabilities, there is practically no improvement at all from det. 30 to det 40 for geforce 2 ultra and lower.
Athlon XP 1.58GHz | MSI KT3 Ultra2 | 256MB DDR PC-3000 | GeForce 4 ti4200 64mb @ 310/533 | IBM 120GXP 40.0GB | Det. 41.09 | DX 9 RC2 | Win XP SP1