HI,
There are many posts on this board discussing what books most people use for USMLE--you can just do a search. I think the general consensus is the High Yield books for anatomy, neuro, and embryo, BRS for path and physio, and Micro Made Simple for micro.
Regarding Biochem and molecular bio, many people say use the Lippencott book. I disagree with this, as I found this book too dense and detailed for boards review. I also didn't really like High Yield biochem, I thought it was too "bare bones." If you can get your hands on the Kaplan Biochem book, I thought this was an ideal balance.
The good news is that Biochem is not really covered on COMLEX, and is not even a big part of the USMLE. I would not worry about memorizing details and reactions and obsure diseases. Stick to big concepts and control points of reaction pathways. In truth, First Aid would have been fine for biochem on USMLE. There were some questions that required you to interpret results of experiements--these are difficult to study for, you just have to know the concepts.
This is also true of the exam in general. A mistake I made was in trying to memorize all sorts of tiny details--the USMLE focuses more on concepts and how to apply them. I didn't really have any questions on obsure diseases--just stick to the basics.
I think the best source for USMLE is Q-Bank. Try to do as many questions as you can. The interface is exactly like the real thing, and many of the questions were so much like the real test it was scary. Many people say that the questions on Q-Bank are more nit-picky than the test, and this is true, but many of the questions are right on.
Regarding when to take each exam, that depends on your schedule. Most people try to take the USMLE a week or two after the COMLEX. If I had it to do over again, this is what I would have done. I felt too burnt out, so I postponed USMLE until middle of third year.
