You can do collision with x files but you'll have to make sure that it's lots of small x objects. The reason for this is that you can only detect if you've collided with the whole object and you don't really get to find out which part. For this reason having a bit of terrain with a building on it as one x model won't work as, if you're on the terrain, you'll always be inside the humungous colision box for the whole thing. Therefore you need to divide the scenery up into individual hills, buildings and suchlike).
A good trick is to use air-lock type inner and outer doors on buildings so that you can show and hide the interiors and exteriors as you go inside a building. This is trickier in fantasy games as thatched cottages tend not to have airlocks!
This is why many people try and use BSP for terrain and just use x for pickups like medikits, guns and other in-game interactive objects.
However, given how dodgy the BSP is at the minute it's worth investing time in x.
Ending a sentence with a French word is so passé
