Wow! Short and great explanation, Xeven. Just to add some info, the DMA option is available not only under cdrom, but also the hd settings. If your devices support dma, use it. Mainly in your hd. Most hds support dma, but there are different versions which gives you different transfer speeds(these are most for hds, since cdroms doesn't transfer at such a high speed):
UltraDMA2=33MB/s
UltraDMA4=66MB/s
UltraDMA5=100MB/s(the latest one)
Your BIOS must have dma tranfers enabled also.