The Rage Pro Turbo does assist in decoding DVDs, but it's a very old chipset which probably uses much less efficient decoding. Your system seems kind of on the borderline of whether you should get a hardware decoder or whether the Rage Pro Turbo will be enough.
If it's integerated, I'm also wondering if you have ATi's DVD player on one of your system discs (I have a feeling that other DVD players may not recognize and utilize the old Rage Pro chipset).