For folks with DMA problems on RH 8.0 (ie. choppy playback of DVDs): If hdparm -d 1 /dev/hdX doesn't work, then try putting following line into /etc/modules.conf:
options ide-cd dma=1
Then remove ide-cd module and let kmod reinsert it automatically (or just reboot, if that sounds complicated for you). This made things work for me.