What is the best solver a poisson type eqn.

[Christian Klettner]

Sorry that I sent this twice. No subject in the first one.

Dear PETSc Team,
I am writing a CFD finite element code in C. From the discretization of
the governing equations I have to solve a Poisson type equation which is
really killing my performance. Which solver/preconditioner from PETSc or
any external packages would you recommend? The size of my problem is from
~30000-100000 DOF per core. What kind of performance would I be able to
expect with this solver/preconditioner?
I am using a 2*quad core 2.3 GHz Opteron. I have decomposed the domain
with Parmetis. The mesh is unstructured.
Also, I am writing a code which studies free surface phenomena so the mesh
is continually changing. Does this matter when choosing a
solver/preconditioner? My left hand side matrix (A in Ax=b) does not
change in time.
Best regards and thank you in advance,
Christian Klettner