I remember playing the 68k MacOS-Version on a friend's 68040/50 Amiga using a Mac-Emulator, was pretty fast IIRC.
Well, I'm just stating from what I've seen on my a4000. I could never even get Doom to run at a decent speed (although it's been years since i tried). If I remember right, Duke3d is mostly Floating Point math. While some Amiga's have FPU, it's not on par with the x86 chips.
As for the Dos emulation in XP, it only works without sound. To get sound you need a program called vdmsound, and that runs like crap on even the lowest settings and a new machine.
I'd agree with Dizzy about playing old DOS games on an older PC though. I'm not really a big fan of old DOS games, but sometimes you need the real thing if you want to get the most out of it. Sure, the PC had it hardware quirks, but so did every computer. If that's something you don't want to bother with, then there's always consoles.