What might help using dvd-playback is XvMC. It is a new special library for mpeg2 decoding and it provides motion compensation. Note that you need a XvMC capable player (XvMC != Xv), not sure if xine and mplayer already have support. (there was discussion about this some time ago ..)
Further your videocard needs XvMC support. All Geforce4 MX and GeforceFX cards have full support including IDCT and motion compensation; Geforce4 Ti cards only have motion compensation support. All other cards (except for the quadro equivalents of the GeforceFX and the Geforce4) lack XvMC support. I hope this helps.