Your system or motherboard usually comes with one DMA100 IDE cable that supports 2 devices. If you have only one CD ROM drive and one hard drive, then you are ok. If you bought one extra hard drive, then you should let both hard drives connected on the fast and more expensive DMA100 IDE cable. And then use this inexpensive DMA33 IDE cable for the CD ROM drive. Your CD ROM drive is most likely DMA33 device anyway. Hooking it to DMA100 IDE cable won't make it operate any faster.