Happy ThanksGiving to everybody :D
Your question about combining the different types of RAM is a very interesting one.
As you probably know, what determines the type of RAM is it all starts at the beginning when it is still a silicon wafer. The dye of these inidividual chips keep shrinking and despite their smaller size are able to store more data, and return the data correctly at ever increasing speeds.
The short answer to your question is `I think so'. At the design level, there is huge competition between those companies involved with RAMBUS and those involved with DDR. My guess, if I were forced to make one would be that before RAMBUS really takes off and sees it's full potential the DDR folks will have incorporated it into their design to negate the competition. (RAM producers stay in business and average of 6 years, very competitive business).
From an engineering standpoint I really don't see any reason why what you are suggesting would not be possible.
Sorry, forgot to answer your A+ question
A+ is a certification that essentially makes you a `PC' hardware expert. It covers all aspects of PC hardware in great detail.
CompTIA is currently the leader (and author?) of the A+ cert track. http://www.comptia.org/certification/ ofcourse they offer some other really neat'O certifications as well.
A good book (bigger than a telephone book and twice as heavy) on the subject is Scott Mueller's 'Upgrading and Repairing PC's, Linux Edition' published by QUE ISBN 0-7897-2075-2. There is a non-Linux Edition that is essentially the same thing. The Linux Edition is more recent, and the only thing that makes it a `Linux Edition' is a pretty in-depth discussion about *nix partitioning in the rear of the book.
I personally think anyone interested in PC hardware should give the A+ study materials a look.
Two great web sites that are totally up on new PC hardware and give details in the extreme are http://www.tomshardware.com/ and http://www.sharkyextreme.com/
Hope this helped