It's a difficult poll in some respects, while those we XBOX 360's and PS3's it can be defined they are compatible with one another, people using PC's or other platforms potentially have a whole host of "configuration" differences.
I would suggest perhaps using some "Benchmarking" systems to identify what can and can't be run for those with PC systems. For instance those running XP version of windows will be limited to DirectX 9.0c where-as those using Vista/Windows7 will be using DirectX 10. (The later DirectX has differences in it's library for extra shader pipelines amongst other things, so some things will only run on the later version)
If you want cross-platform gaming that most of us can handle, then you'll have to look to fully OpenGL games, this means they can be be emulated on Windows, Linux and Mac alike. (Incidentally it's "Mac" for Macintosh, otherwise you might get it mixed up with MAC with could be "Media Access Control" which is the address that identifies a particular connectible adapter.)