As it is often said, there is a lot of confusion out there but personally I think every bit of advice you'll find on PMDG's manuals how to set up your 737 NGX can help and they are really the ultimate experts in that field. They also recommend highly only to use win 7, 64 bit but this is not really the topic right now.
I haven't mentioned yet, that I ran every slider in the graphics section of FSX to the max, except water, which is 2 high, but not very high. Active sky and similar programs do a better job anyway as standard FSX