To enable DMA in Win98, you need to enter the device manager in the system properties dialog. This can be opened in one of two ways:Click 'system' in the control panel, or press the 'windows key' and the Pause-Break key simultaneously.
Once opened, click on the device manager tab, and click the 'plus' sign next to cd-rom. Double click the Cd-rom you want to enable DMA on, and click on the settings tab of the cd-rom properties window that you just opened. Click the box marked DMA, and click ok. Repeat this procedure on your other cd-rom/Cd-rw. It might be a good idea to check to see if your hard drive has DMA enabled while your there.
I hope that helps, and personally I don't think there would be a difference in data copying with either of those 50x drives (the Prime and I/O magic). Audio copying speeds might differ between the two however.
Jake