HOW TO: Dividing World into how many Objects?

Let's say you're building an FPS and a series of obstacles on a terrain.

Objects:
a. Building A
b. Building B
c. Building C
etc...
z. Terrain (Landscape)

You're building those 3D objects in Max or other tool. There needs to be collision detection between the SHOOTER and those objects.

Will you load:
a. ONE .X file with ALL objects
b. ALL buildings as an object, Terrain as another
c. EVERY object alone

What's the best practice, perf wise, etc...

Thx