if your hard drive has any fragmentation on it at all, it wont even be able to burn discs at 16x.......
i've got a 10,000rpm sata hard drive, and unless i create an image and burn it to disc straight away (disc-to-disc copies) my drive can hardly ever keep up with 16x.
just something to consider, since you can get a 16x for $50, and the 18 and 20x are usually a bit more.