Depends on how you interpret "Emulate". Since you are speaking of a dos card I suspect you mean hardware emulation. I run Win98 pretty decent on a Vortex Golden Gate 486 Bridgeboard (16MB RAM/Co-Pro etc).
There are also SBC (single board computers) as fast as Pentium 4 or Xeon you could stick in your Amiga; but this isn't really emulating; there is no interaction whatsoever between that SBC and the Amiga. Using a KWM switch however; it is pretty fun to use a fast Windows XP machine from within your Amiga
