I get an FPS that varies from 25 to 60 for the more detailed sceneries. Is it normal for the FPS to vary that much? Also, should I be getting a better FPS with my settup?

I have a dell notebook with a pentium 4
My video card is a ATI mobility radeon 7500c with 32 Mb

Seems like with this setup I should never go below 33 FPS.

Any ideas?