>What does enabling DMA do?
Allows your CD-Rom to move information directly to your system's RAM rather than having to make the processor shuffle bits back and forth to get it there. As you may have observed, when DMA isn't activated your BIOS will tell you that it's using "PIO" (Programmed Input/Output mode) this means that whenever requests to your CD-Rom drive are made the processor has to go out across the bus, capture the information, then store it in memory. With DMA the processor is merely told that a memory copy has been done and makes a note of it, thereby decreasing the amount of work it has to do and, consequently, gives more of the processor's power back to your applications. (Yes, I'm aware that my explination leaves out a few nuances of the different PIO modes and the like, but lets not get mired in the details.)
>Would you recommend it?
If you know for certain that your drive supports it, enable it and never bother with the thing again. Some VIA chipsets (I'm using an Intel motherboard now, so I can't say if its changed or not) don't allow you to use DMA, but I'm pretty sure using their 4-in-1 drivers (or at least the bus mastering controller update) will give you an equivalent result.
>Are there any drawbacks?
If your drive *doesn't* support it, doing a read from the drive can either lock up your computer, give you a BSOD, or otherwise. It's nothing dire though, a restart will get you out of it more than likely. Then you can just disable DMA, restart, and things will return to normal (well, as normal as Windows can be

).