The problem with using a .x file in this community as your user base, is that not many people are familiar with Direct X file formats. Additionally, that would prevent the end user from sharing their creations with a MAC user unless they have a tool to convert from .X to .OBJ. I'd suggest looking for documentation on the Wavefront Object format or the .3DS format. Both of these are pretty useful and can be imported by just about any 3D package under the sun.