I have the same computer as you and here was my experience:
The P2400 worked fine on DVD's with my first videocard and decoder card until the matrix came out. I then got a voodoo3 3000 and used dvdwin for software encoding and it still sucked. The scene when the helicoptor crashes into the building as well as a few other intensive ones just crashes the machine.
So I said screw it and bought a home entertainment system
As far as motherboards with integrated dvd decoding they don't exist, but the integrated video on some of the new boards is supposed to run dvd reasonably. I don't know which ones are good for your p2 400 since I didn't worry too much about it when it didn't work. Nonetheless, I doubt the integrated video would be better than your video card, so just go get dvdwin or some other software decoding software and see how you do.
"I think I brained my damage"