I will try to address the questions. I hope I can completely answer all questions.
As far as as the Capitol Box Volume 3. These new releases are not re-releases in any way. The Capitol Boxes were different mixes than what is on the 1987 CDs. Different in several ways. Different track order, different albums. The Capitol Boxes were the American versions of the albums. The new remasters are the UK versions of the albums.
We probably will never see a Capitol Box Volume 3 because from Revolver on the mixes were basically the same as the UK versions. We are not getting LIB - Naked. This is a remixed version of Let It Be. That album stands on it's own. It is not part of the Beatles canon, per se.
The difference between the mono and the stereo is dramatic. In 1987 when the CDs were originally released it was decided to release the first four albums in mono. The rest were released in stereo with Help! and Rubber Soul being remixed.
When the albums were originally recorded and mixed (up to 'The Beatles' (commonly known as the White Album)) the most attention was paid to the mono mixes. In some cases there were actually different takes used for stereo or mono releases. In other cases the edits were different, one case is Sgt. Pepper's Lonely Hearts Club Band (the song) the transition into With A Little Help From My Friends is different. The mono version having a slightly longer edit and some different sound effects. There are more than just what I have listed.
I have not heard the remasters yet, but I do have both the mono and stereo versions. I too, enjoy hearing subtleties in the recordings, and I look forward to hearing new things with the remasters. Everything I have read, is that the remasters should live up to expectations.
Again, I hope this answers all of your questions.
I'll address a few of the other questions, and clarify on some of Gary's comments.
These releases only cover the music that was released by the Beatles through May 1970, that being the original Let It Be album. However, it does cover all of the released tracks of The Beatles while they were an active unit. It does not include any compilations (MMT not withstanding), only the original 1960's releases.
The only way to get the mono recordings is via a boxed set which costs around $250. Unless you are willing to part with that kind of money, you'll be acquiring the stereo re-masters. (The reason for this is the the mono recordings are being marketed towards the hard-core collector and student of their music.) If so, then you're free to get individual releases over time and you should get them all.
That said, if you can come up with the mono box, I would highly recommend that you do. First of all, it's a limited edition, so if you decide you want it later on, it will most likely cost you a whole lot more that the $250 you'd pay now. Secondly, these are in my mind, the definitive versions of their music. It is well known that these are the versions that the Beatles themselves oversaw the mixing of.
As far as how I recommend listening... the cheapest way to get the highest possible quality listening experience is by investing in a good pair of earphones. Do your research and then buy the best set you can afford. Personally I use two different sets of earphones for when I want to do critical listening. One is an ever-the-ear Sony MDR-V6. These are big and completely cover your ears, but they sound AMAZING and are comfortable for extended periods of time. However, they are not practical to use as a portable headphone. You should be able to find them for about $70.
I also have a pair of Ultimate Ears earbuds, that go inside the ear canal. These are also amazing to listen to, although not as comfortable as the big headphones for periods of multiple hours. They'll also set you back about $250.
Of course, the quality of your headphones is irrelevant if you're not listening to good equipment. The iPod sounds great, but if your really into getting the best fidelity possible (which you are if you've already gotten those headphones), make sure you convert the CDs to LOSSLESS files, so that the resulting music sounds identical
to what is on the disc.