I tried out GMAX a while ago just out of curiosity; it is actually almost the same thing as 3DS MAX as far as modeling goes. The main problem is the proprietary file format, but someone out there might have made a gmax->3ds converter.

The only FPS games I have created extensive modifications for are Doom and Descent 2. Doom was my first experience into modding, after which I got very, very heavily into modifying both Descent 2 and C&C: Red Alert (an RTS game) for a few years and learned a lot about how games in general work. After that came Freespace, and then of course, Freespace 2.
