Its a 97 540i 6 speed . The things I already have done are:
K&n intake . Performance muffler. Removed resinator.

I already Know about the Dinan , so if you know anything else that can help me to improve HP ( cheap) let me know. Thank you .

You can do install a good turbo or supercharger.

OR

You can lose some of those weight on the car to get a better power to weight ratio.

Nothing really. You could probably swap in a modern m3 engine (v8)...but it would cost more than your car is worth.