Ok. So, based on comments above I now have a revised list of machines to build. I've added a machine to cope specifically with older games that the Pentium MMX @ 300Mhz is too fast for. The new machine can be slowed to less than the speed of a 386 I think (Possibly as slow as a fast 286). I've removed one of the Win98 machines, and shifted around what GPUs are used, and the CPUs paired with them.
This is the updated list:
DOS 6.22/ Win3.11 (1985-1990) - Psuedo 386/25mhz-486/25mhz
(L2 Cache disabled. SETMUL to add further scaling options)
Pentium MMX – 166Mhz @100Mhz
8mb EDO RAM
ISA Trident 8900C 512mb
Sound Blaster Pro 2.0
Micro ATX Desktop PC
DOS 7 / Win95 (1990-1997)
(Can slow to the speed of a 486 by disabling caches and using SETMUL)
Pentium MMX – 233Mhz @300Mhz (Jan 1997) on Super-Socket 7 motherboard
64Mb PC100 SDRAM
AGP Riva 128 4mb (April 1997) + Voodoo 1 4mb (Oct 1996)
Soundblaster 16 CT2230
Horizontal ATX Desktop PC
DOS 7 / Early Win98 (1997-1999)
Slot 1 Pentium III – 550Mhz (Apr 1998)
128Mb PC100 RAM
TNT 2 (Oct 1999) + Voodoo 2 SLI (Feb 1998)
Sound Blaster 16 CT2290
Large Tower ATX Desktop PC
DOS / Late Win98 (1999-2001)
Socket 370 Pentium III – 1000Mhz (Oct 1999)
256Mb PC133 RAM
Geforce 2 GTS 32Mb (April 2000)
Sound Blaster 32 CT3670 (in DOS only) + Aureal Vortex 2 (In Windows Only)
Horizontal ATX Desktop PC
High End Win98 (2001-2003) / Early XP (2001-2003)
Pentium 4 – 3.4Ghz Cedarmill (Jan 2006)
512mb DDR RAM
Geforce 4600 (Feb 2002)
Sound Blaster Live
Micro ATX Desktop PC
Early WinXP (2001-2005)
Core 2 Duo E6600 (Jul 2006)
2Gb DDR RAM
Geforce 6800 128Mb (April 2004)
Sound Blaster Audigy 2 ZS
Micro ATX Desktop PC
Late WinXP (2005-2010) / Windows Vista (2007-2009)
4Gb DDR2 RAM
Core 2 Quad QX9650 @3.8Ghz (Nov 2007)
Geforce 8800 GTS 512 (Dec 2007)
Auzentech Prelude 7.1 Sound Blaster X-FI
Large Tower ATX Desktop PC