Not so fast - while that's not necessarily the wrong answer, let's muddy up the waters a bit, shall we? Depending on the year of manufacture, a given car may not be equipped with many modern emissions controls that are standard on current models. Even if a car continues to pass its emissions testing, it is generally only required to meet the standards that were in place when it was new, and those requirements have gotten much more strict in the past few years.
Scrapping a car, as pointed out by SciAm, isn't a particularly green option, though retrofitting modern emissions equipment is tough. A conversion to electric sounds like a nice option, but then you must consider where the electricity is coming from. As you can see, this is anything but a cut-and-dry answer, and it depends largely on what green aspect you consider most important. To keep or to buy new, that is the question. Thanks for the tip, Micah!
[Source: Scientific American]