After reading through the exam objectives and threads, I have come to the conclusion that these exams require a lot of resources to truly learn the material and that they could be studied together, as the material complements each other. I have gone through the MS Press 640 and labbed everything I could, except for Federation Services - can't run 6 servers on my workstation

I am currently going through the Sybex all in one, reading through the 640/642 sections and while it covers the material fairly well, the hands-on leaves a lot to be desired. I am getting ready to start the Server 2008 R2 Unleashed next and just skimming through it a bit, it looks to be a solid resource. Technet has been invaluable for filling in some of the blanks, especially with AD LDS and RRAS. I have also typed out some notes - 30 pages+ from all the sources I've used and read through those a couple times per week.
I did the 680 before starting the 640/642 and interestingly enough, a lot of the concepts from that point of view have made the 640 material easier to understand and probably vice versa.
Another method I've used is to go through the exam objectives and make up my own lab scenarios. This has helped cement the concepts and I'm hoping that if I can walk through the steps of configuring DNS, DHCP, RRAS, AD and it's various roles in my head, I should be well on my way.
Good luck to those taking the 640 and 642 in the future!