Yeah, collision can be checked for between a .x model and another model. Any object in the 3D world of DarkBASIC can have a collision check on it. That means that, any model that is loaded in, any object that is created internally ( ie, using the Make Object Whatever commands ) and Matricies can all have collision on them.
The only difference between using a .x model as oposed to a matrix for terrain is that you can't use the Get Ground Height function and others that specifically relate to matricies.
You have to code your own Functions for these ( but that's pretty simple once you know how ).
If you are to use .x objects for the levels etc, then by all means, get NG's Collision DLL. ( I personally haven't used it besides the demo, but ) It's a great system, and with all the extra, super fast collision commands that it allows you to use, it's well worth the small fee for the latest version.
Hopefully that'll enlighten you on the situation.
Jess.

Team EOD :: Programmer/Logical Engineer/All-Round Nice Guy