Well, for this particular building, i'm not using a terrain as such, just the normal grass segment. However, i can make terrain (see pic).i'm getting poor frame rates too but my PC is old so i think i need an upgrade.
Possible ways to solve Lag (low frame-rate):
Hardware:
Get an up-to-date graphics card
Get an up-to-date Video Card
Get as much RAM as you can (1gb+ ideally)
Get a faster/better processor (Athalon is better than Pentium)
Software:
Make sure the latest version of Direct X is installed (9.0c, i think is the most recent)
When testing a game, turn off all other applications
Level Design:
Make all entities that don't need to move static
Don't have lots of big open spaces
Don't have lots of long corridor, put a doors in them to block polys
Don't have lots of entities in one room
Don't put the entire US army in one room
Changing some of these should help,
AE