I don't have a "Dream Build" per-say but I have two systems which pretty much are my go-to systems that don't get dismantled. They may change slightly over time but the base always stays the same. These systems fulfil the requirements in need to play the games I like (DOS/Win98).
My first system is my K6-III+ machine. This it the machine which is always plugged in. It's had a few upgrades over the course but this is my longest running machine.
AMD K6-III+ 400 @ 500
Asus P5A-B v1.03
128mb SD-Ram
3DFX Voodoo 3 3000 AGP graphics card
Matrox M3D Power VR PCI Card
Sound Blaster Live!
Yamaha YMF719 with Dreamblaster S2
Terratec EWs64s With 32mb Ram
FSP 250watt PSU
AT Case
2nd one is my Tualatin PIII 1400-S Machine (This is currently being re-housed into a tower case) (Old Photo)
Pentium III Tualatin 1400-s (Korean-guy modded) @ 1470mhz (140fsb)
DFI CB60-BX Rev C1 Intel BX Chipset
512 Crucial 133 CL2 Ram
Nvidia Geforce 4 Ti4600
Diamond Monster MX300 (Yamaha DB50XG)
Turtle Beach Malibu
Awe 64 Gold
3rd ISA slot gets rotated with different cards
These two machines pretty much cover everything I need in regards to the games I play so I would class these as my Dream builds. I have a bunch of Midi Modules to go with these machine (MT-32,SC-155,NS5R) so really when it comes to usable hardware which I would need to play games I think I am pretty much covered. Oh and I run these on a nice 19" CTX Trinitron CRT monitor