Here is the system I would build for the games you mentioned:
CPU: AMD K6 or Intel Pentium MMX (somewhere between 166 and 233 MHz)
I'd prefer the K6, because it has no locked multipliers and can be clocked down to 100 MHz if needed. Note that some games reaching back to 1990 may have problems with Pentium class CPUs, not only because of the higher clock speed, but also because they sometimes used non-standard 386/486 tricks which will crash a Pentium.
Board: A brand name Socket 7 ATX board with the Intel 430TX chipset.
The TX is one of the least problematic and most versatile chipsets I've ever used. The FSB range is 50 to 66 MHz (some boards go as high as 83 MHz). Supports SDRAM, UDMA on the IDE controller, and the ATX versions often come with USB and PS/2 connectors.
Memory: 64MB SDRAM
More than that wouldn't be cached anyway. Plenty for DOS and about right for Windows, if all you do with it is playing games.
Graphics: 3dfx Voodoo Banshee
Super-fast framebuffer and Glide support, very compatible with older games. Works well with UniVBE.
Sound: ISA SoundBlaster 16 or AWE32/64. Wavetable card recommended.
Storage: 30 GB harddisk, CD-ROM drive (16x minimum)
OS: Windows 95 OSR2, using the integrated DOS7 for DOS games