Well for games that don't use Direct3D (d3dim/d3drm.dll) there's always wrappers to set compatibility flags in ddraw, or a rewritten ddraw.dll such as DXGL

There's no general purpose utility to do this yet. Wrappers are game specific or limited. DXGL works ok on some DirectDraw games, but there's no sound for example, it's really early in development. Maybe years from now it'll be a good solution.

If you go to the extent of building a rig just for older games, you can safely install up to DX8.1 on a 9x system without worrying about DX incompatibility provided you use an older graphics card and period correct drivers. Windows 2000 is pretty forgiving overall, basically the same deal. And with XP you pretty much have to stick to SP2 and sometimes use older drivers (even though you're stuck with DX9). Game specific patches will help; but you might as well just use Win7 if it ran OK in XP.

I'd stay away from any NT-based OS since they can't set palette index 0

Yeah, the whole point is trying to delineate what games each of my older machines will cover. Anything that runs ok in DOSbox and doesn't need real MIDI or Joystick support gets run on my modern rig. My p200 system with Win95C gets the older stuff that needs a voodoo1. The p3 98 box gets everything that won't run on XP. Anything newer gets run on the modern rig.