Reduce dof's in FEM

Hi 

  I have MDOF equation:

                MX"+CX'+KX = F(t).

 Say the order of each matrix is  nXn. I want to reduce the order of the matrices. 

I will find the eigen vectors and get modal vectors matrix phi. I will pick some dominating frequencies say around 24 modes.

then reducedM = phi'*M*phi.

Now reduceM has an order of 24X24. Will this procedure give me satisfactory result at last? or are there any other methods to reduce the dof's.

Thanks in advance...

Sreenivas