[The Fifth Horseman]

For me, that would be:
- 486 DX4/100 to Pentium 133 Depending on what you can get, really. Won't make much difference, and you'll still have to use slowdown utilities for some older games.
- 8 to 32 MB RAM, EDO chips if you can acquire any EDO memory has about 5% greater speed than that of regular SIMM modules and outpaces regular DIMM memory on FSB speeds up to 80 mHz. 8 MB is usually more than enough for most DOS games you can run on this setup, and going over 16 may only have sense if you want to use the machine for Windows gaming as well. Note that some games (eg Space Hulk) may refuse to run on systems with over 24 MB of RAM.
- 1 MB graphics card would be absolutely sufficient for DOS gaming, and more than adequate for most Windows games you can run on that setup. For the few where it's not enough for smooth graphics (and smoother video playback), a 4 or 8 MB card should be easy to find.
- ESS 1688 soundcard Sound Blaster Pro compatible, and all in all a great card. Not neccesarily the best one out there, but popular enough to be easy to find if you can't find an actual Sound Blaster 16 like Scatty suggested
- 512 KB cache Many of old motherboards offered some way to increase the amount of available cache memory - the default was usually 128 to 256 KB, with some motherboards supporting up to 1 MB. 512 KB will give the system a nice boost of speed, especially if you run Windows on it.
- Windows 95 OSR 2. It's better than plain W95, since it has FAT32 support and some rudimentary USB compatibility. Windows 98 is an option, but you need at least 32 MB of RAM for it to run at any acceptable speed.

Regarding sources for old hardware... look at eBay or nearby garage sales. You can find great pieces going for ridiculously low prices there.