I feel Princeton Review
are the two tests you should take. Princeton gives you an estimate on the higher side by about 30 and Kaplan
(which has far more difficult math) gives you the lower bound by about 50-100.
Powerprep (ETS's S/w) is a good simulator, the questions on the actual GMAT are almost as easy/tough.
cycle yourself through these tests. Take a Kaplan
then a Princeton. Save the powerpreps for the final two weeks. They were the best indicators for me. (+/- 10 to the actual score)
If you've studied for four weeks, take a few days break and honestly review your strengths and weaknesses. Don't bother yourself too much with the simulated tests, its the real one that counts.