Hi-
For general usage, there is no real benefit to be had by using other than combo burners.
As you have seen, DVD-R speeds of 16x, 18x, and 20x are common place now. Because of this, media has also changed to accomodate the faster burning capabilities.
This is a main issue to be considered, because many problems are typically media related. The newest media has difficulty on older burners, so the first advantage of a new burner is to gain reliable compatibility with new, faster rated media.
Installing a new 18x drive, though, doesn't necessarily mean that you will be able to burn that fast. In your machine, good, reliable burns, will typically max out at 10x, with 12x being possible, 8x the most capable and reliable. This is simply due to the architecture of the older machine.
Does this mean that you should only get an 8x capable burner? No. One, there are few slower models available, and the media compatibility issue will be a problem.
In choosing a model, your most pressing concern is compatibility with your software. With Tiger, 10.4.6 and beyond, all the Pioneer models have native support. In Leopard, support has been broadened, as well.
Burners from Pioneer, LG, Samsung, Sony, among others, are the most commonly referenced burners. For reliability, compatibility, and proven performance, I can easily recommend the Pioneer DVR-112.
If you have interest in what others are using, in machines similar to yours, as well as user reports, check the
Xlr8yourMac Drive Compatibility Database.