Funnily enough I've just reviewed all the software based DVD players for a magazine. The best by a long way is SoftDVD Max 4.0. However - do bear in mind, that whilst decoding quality is excellent, with a rig like yours you're going to suffer buffer under-runs and consequent frame skips, even if you have got a high-end DVD-ROM drive. Even on my dual PIII800 rig with a new Pioneer DVD-ROM drive I was experiencing skipping - on a Toshiba Satellite laptop I tried it on (Celeron 600 based), it was virtually unwatchable. The only concrete way of getting solid playback is to invest in a hardware decoder like the Hollywood Plus.
