Ok, first things first, the title of 1.2.1 should be 486. Not Intel 468, because AMD also produced 486 CPU's.
Also, there are the AMD K5's. Basically their Pentium equivalent.
The 486 & Pentium days were before the manufacturers started using proprietary socket standards, so a K5 will fit where a Pentium used to be and vice versa. The only thing that needs changing is the jumper setting.
Also, from what do I know the AMD CPU's of that era handle overclocking much better.
What do I suggest for 486 is 486 DX4/100. That's more or less equivalent to Pentium 90, but can run things meant for P133 without any noticeable slowdowns (been there, done that).
For section 1.3 (RAM), I'd like to say that - from what I heard - some 486 machines may also require paired modules. In all cases, the best idea is to check motherboard manual to confirm that. My own Shuttle HOT-433 motherboard did not require paired modules, for instance.
Some early Pentium boards may not handle larger DIMM modules - eg it may only show 8 megs of the memory, even if the module is several times larger.
Also, when I say "paired" modules I mean identical modules from the same manufacturer, not just same size modules. Without paired modules, the computer may work, but you will experience random crashes and memory corruption.
Graphics cards... well, if you want DOS games you'll want an SVGA-compatibile one. 1 MB memory should be right enough for nearly all DOS games and some of the Windows ones, altough you can go anywhere up to 8 MB (Windows games that make use of Smacker Video encoding for cutscenes may experience glitches in the cutscenes on cards with less RAM).
NOTE: some of the larger cards may not be compatibile with 486 motherboards. A perfectly functional ATI Rage XL did not cope with - also perfectly functional - Shuttle HOT-433 motherboard. So unless you are sure you'll need a better one, stay with 1-meggers.
Sound card: It should be noted that there was a type of soundcards called "Roland LAPC-I" (that's large "i", not "1"). These babies were for ISA slot and pretty huge in terms of size taken inside the machine, but had an onboard integrated Roland MT-32 midi synthethiser.
Hard Drives:
For DOS, just almost anything. 200 MB should be sufficient, if not satisfactory. There is little point in going over 1 GB.
For Windows, 800 MB or higher (or 800 MB single partition).
It's a wise precaution to make sure your hard drives are SMART-capable and your motherboard can read the SMART information from them.
To make long story short, SMART is a monitoring technology that allows you to monitor the status of your hard drives. Most old HDD's break down with time, so it becomes very important to know whether yours are on their way to scrap heap or not.
More on SMART here.
WARNING: I advise against using "Low-Level Format" option present on some motherboards. This can destroy or damage the HDD.
Mice: About any COM or PS/2 mouse will do for DOS usage. For Windows, my personal bias go to A4Tech 4D+ and 4D++. Both are three-button two-wheel mice for PS/2 that operate nicely with a COM adapter (often bundled).
Cache: In the past, motherboards had removeable/upgradeable L2 cache. Older x86 boards had a number of sockets with cache chips (looks similar to the BIOS chip, cept it's smaller and there are several of them in close proximity), most Pentium boards have their cache memory integrated onboard and usually sport a cache expansion module slot (similar in appearance to a PCI slot, but placed away from the expansion card slots). 256K cache will do (128 also should), but I advise you try to find means of expanding to 512K or 1024K, because it can result in a significant performance boost.
For OS, there are following choices:
DOS:
* MS-DOS 6.22 : Last "true" MS-DOS system. Not compatibile with FAT-32.
* MS-DOS 7.1 : A version of DOS someone hacked from Windows 98. FAT-32 compatibile.
* FreeDOS : Freeware DOS system, FAT-32 compatibile.
In theory some versions of DOS can be forced to cope with USB devices like memory sticks. (there should be more about that in that oldschool computing site I posted about in the "Vintage Computing" sticky).
Generally, DOS alone isn't the best idea. A second OS is advisable, either Windows or some sort of Linux. This is because there are better and more up to date programs for file management, archive operation and hardware monitoring available for later systems then for DOS.
Windows:
Windows 3.x is pretty much useless, with barely any advantage over DOS with a proper GUI. Windows 95 is advised for high-end 486's and low-end Pentiums. Windows 98 is advised for most Pentiums over 100 Mhz with sufficient RAM, altough it may lag a bit.
* Windows 95 OSR 1: Earliest Windows 95 version. Not FAT-32 compatibile.
* Windows 95 OSR 2: Later Win95 upgrade. FAT-32 compatibile.
* Windows 95 OSR 2.5: Last Win95 version to exist. My sources differ as to whether this is an original Microsoft product or one modified by some hacker. Similarily, they differ as to whether it is compatibile with USB memory devices or not.
Note: Windows 98 just won't work on 486 architecture systems.
* Windows 98: FAT-32 compatibile, compatibile with any USB devices if proper drivers are installed (manual driver installation required for each USB device)
Linux:
Don't know that much about Linux OS'es, so I can't tell you much about them.
* Damn Small Linux: Bootable CD or bootable floppy that starts the CD (in case your machine is not compatibile with some types of bootable CD's; can happen, had that myself). Can install on your hard drive or run off a ramdisk (portion of your RAM reserved by the program and turned into a virtual hard drive) if you have 128 MB or more. Not sure how much system resources it needs to operate with full GUI, the basic command line should work smoothly even on a 486.
I'll type in some more on suggested software later.
The Dual Boot section should be renamed to "Running multiple systems on single computer" (and for a reason), with two sub-sections: "Boot Managers" and "Hardware-based methods". I can provide pretty much the entire content for the latter (which can be done easier then setting up a multi-boot system, though not as neat and clean in the end result).
Also, I think that the guide could later use a section on "Methods of transferring data to and from the DOS machine". There are many things that can be said about that, and some interesting points for less tech-savy users.