@ Alduce: I suppose you mean "questions" instead of "answers"!
My answers to your questions are:
1.) In the actual version of DarkPhysics(=DP) you only need the special hardware-card for the fluids! (This may change in future updates, maybe every feature might be possible in the future without the hardware, BUT slower then running in hardware-mode!) All other commands do work without it in software-mode!
2.) You only need to install the AGEIA-Drivers from their website to use/run programs/games that use the AGEIA-PhysX-SDK, like DP does! If you only try to compile some code with Dark-Basic-Pro (=DB-Pro) without having DP then it won't work! Demos that come as an EXE do run as long as you have the latest AGEIA-Driver installed (if they don't run fine, there might be some other errors that cause the problems!)!
3.) You can use other physic-engines with DB-Pro like Newton:
http://forum.thegamecreators.com/?m=forum_view&t=50438&b=5
Programming a physics-engine by yourself might also be possible, BUT is a lot more difficult and time-expensive then useing an engine/SDK that already exists!
Next time you might want to use the search function, cause nearly all of your questions were answered here in the forum before!
I hope this answers are what you were looking for!
Hoozer
