First thing is to lower shadow quality. It doesn't really adversely effect the visuals but it makes a big impact on resource usage. Also, try turning anti aliasing options lower, or off entirely. in the game itself you can try things like turning character shadows off, the dust effects (its only from footsteps) and turning extended drawing off too...

If those don't help enough you can also turn off the physics effect, though that tends to make everything look very flat (as does the character shadows, tbh.)

You might also look into upgrading your RAM. you can probably get 4-6 gigs without too much of an investment, or 8 if you want to splurge a bit.