How to make FSX scenery run better.

[Excalibur372]

I have FSX and I have pretty good scenery. Enough to where at the atlanta airport almost every gate has a jetway. That's not what I'm worried about. In large cities I want to have a pretty good amount of buildings. when i put autogen to normal the framerates lock up and get slow. is there a specific amount of space I need to have? Please Help!!