Awesome thread, i love retro pc gaming.
For DOS i do not use MS-DOS unless i was to keep a pc original, Free DOS is better as it supports much more like larger hard drives and memory.
My main legacy pc game rig specs
OS: Windows 98 se with unofficial sp3 update
CPU: AMD K6 II 500mhz
RAM: 512mb sdram
Motherboard: DFI K6XV3+ (super socket 7) has AGP 2x, ISA and PCI slots.
HDD: 120gb (cant remember the model)
Video/graphics: Nvidia geforce 4 mx420, 2x voodoo 2 12mb sli
Optical Drive: Creative 8x DVD
Floppy: 1.2mb 3 1/2", 320kb 5 1/4"
Case: Unknown brand of ATX case
PSU: Seasonic 350w ATX (i think its 350w)
Sound: Creative Soundblaster AWE64 Gold, Yamaha MU50 external midi. Edifier stereo speakers (which i use with my retro gaming consoles and home computers like the Amiga/Ataris).
PCI cards: Realtek LAN, some USB 2.0 job that was the only one i got to work with the pc.
KB/M: Anker Mouse 5000dpi (overkill but it was cheap, random logitech serial mouse, some generic AT rubber dome keyboard, Corsair K90 usb Cherry MX Black Keyboard.
Monitor: big ass 21" 4:3 Sony Trinitron.
I have another rig which is baby AT form factor; its in bits but it has a Cyrix 5x86 100mhz cpu with 128mb of ram, voodoo banshee graphics card. Can't get it to boot past the initial bios, working on getting it to boot from a HDD.