I'll third the PMDG 737NG add ons.
Watch out for the PMDG 747-400 which is due out anytime now. Got a great review in a recent PC Pilot article.
Other sites to look at are:
Avsim
IFDG
Project Opensky
Simviation
Surclaro
I'm sure there are more but I'm very busy at the moment.
Have fun!