hi
In order to improve your frame rate you can use clip_range which roughly speaking determines the distance at which the engine stops drawing. You should read the manual on this it will give you a more accurate description.
In order to avoid the person who is playing your game from seeing large amounts of the level disappearing you can use the fog to cover it.
You can also use LOD (level of detail) whereby you can determine the level of detail of an object via wdl depedning on its distance in realtion to the camera. However in order to do this you will have to create versions of the object with lower polygon count. Again the maunal would give you more information on how this is used.
Information on how clip_range, LOD and fog can also be found in this forum by using the search I think there is quite a lot of information on it.
These things are mainly, although not limted to, outdoor levels. With respects to the camera only drawing what it 'sees' I think the engine does this already. If it is an in door level you will have to take care of polygon count, note that a lot of detail can be created using sprites and textures. Also note that when using textures it is important to use an appropriate size to the object you are using it on. Using a small texture for a big block will increase the number of polygons. However using a large texture for a samll object will take up to much texture memory. Hence you have to strike a balance.
Hope this helps
Dragon100