My take, aside from a modern system it can be done with two:
K6-2/III+ or C3 Ezra (for DOS)
S478, S754, S939 (for high performance DOS, W98, and WinXP titles)
- should have a downward unlocked multiplier
- preferably equipped with an AGP slot
- ideally using an SIS or VIA chipset for driver support and more DOS PCI sound compatibility
- can be equipped with a PowerVR PCX2 and 3dfx setup for additional graphics compatibility
- usable for difficult games of the era, like Splinter Cell, with the right graphics card

No Windows 10. Stick to 7. The compatibility is better and you don't support Microsofts data mining BS and there forced updates and other forced garbage.

DoktorSatan wrote:

Are there any games that these four operating systems don't cover?

The operating system is not so much a concern as the underlying hardware. Just about any DOS game should run in the version of MS-DOS provided with Windows 98, but any hardware capable of running Windows 98 will likely be much, much too fast for the oldest CGA games.

Is dual booting or emulation worth it? I would like to be "practical" and take any shortcuts available to me.

Good! Stick with DOSBox. It is entirely adequate for the overwhelming majority of DOS games and is immensely more practical than filling your living space with computers. PCem is also popular these days for Win9x games. XP running in VMware should let you run any 3d-accelerated game for Windows XP on any computer capable of running VMware.

The next question is to decide what you actually want to play. Playing "any computer game" is inherently impractical, and fixes are often available to get many popular old Windows games running under XP and/or 7. If you strive to "guarantee 100% compatibility" then you will be pursuing that goal for the rest of your life and will probably never get around to playing "any computer game".