Definitely sit tight, the next years edition publication is imminent. In fact, the AAMC says it's sold of 2007-2008 edition but You might even be able to pre-order it from aamc.org. Regardless, thats probably the best place to buy it.
Is it necessary? It depends on how much of a perfectionist you are. You can gleam a lot of information online (the AAMC has a list of admissions pages here) and the MSAR is frustrating in that it lives up to its name, it supplies really only admissions requirements and dates which will be a minor part of selecting a school. Its information on the last matriculating class for each school is pretty edifying, however. For instance, I learned its much easier to get into Ohio State as a nonresident applicant than U Michigan (statistically speaking, that is).
For me, Every city library I've ever been inside has an MSAR and probably MCAT books as well. As miserly as it sounds, I've found that sufficient enough for me to save over $150 dollars so far.