hell yeah, you can do a lot of things with it.
first off, you're probably not going to run Windows on a machine like that if you are doing emulation. Pure Dos is the way to go.
After that there are plenty of old emulators that should run just fine. You should be able to get 100% speed out of some classic arcade games in MAME, in NeoRageX, Snes9x, and maybe one of the older NES emulators like RockNes or something. Sure, the emulation might not be 100% accurate, but it's still a lot fun.
Also, if you must run an OS, try a stripped down version of Linux, it should be able to run much faster and perhaps allow you to compile one of the newer versions of MAME for greater accuracy. It might even be a little faster than dos in some situations.
Plenty of life in those old machines if you just have a little patience and know how.