Sheri,
I would have to vote for waiting until August for many reasons:
1. Taking the August MCAT does not set you back an entire year in the application process. And waiting until August means that you will actually be prepared to take the test. (Also, once you take the darn thing, believe me, you won't want to prepare for and take it again.)
2. You have to remember that the test is scaled, so that how well you do is dependent on how everyone else who takes the test does. So if most people have already taken organic (and most have) not having the class puts you at a distinct disadvantage.
3. Despite what Mire says, there is a lot of organic in the bio section, lately the percentage has been increasing. Some tests have 50-60% organic. And confidence is a huge factor in MCAT success. If you open the first page of bio and see a complex reaction mechanism and freak out because you've never seen anything like it before, your score will probably suffer.
The whole med school application thing takes time. It takes forever. Unfortunatly there aren't a lot of shortcuts. The MCAT is an important test and it's not easy. Going into an expensive and important test planning to guess doesn't really make good sense, especially when there's no real reason not to wait.
Finally, have you started preparing for the exam yet? Since it's March and you haven't taken organic, if you haven't started studying (or taking practice tests) yet, you might not have enough time to catch up.
So my advice (also a late 20's non-trad) is to wait until August and slam the thing!
Good luck!
BTW As far as taking the practice exams from AAMC, I found the one I took (V, I think) REALLY easy compared to the real thing.