Pretty sure a P4 fileserver with a decent NIC will outperform the Pi several times over. The USB bus on the Pi is sloooooow.
I have a p4 preshot that I use as a Frankenstein dos machine haha it's PGA iirc.. as for the celeron, pump 3v into it and use it as a mini stove or coffee cup warmer about all it's good for haha I like to call the celerons celery because that's how much power they have hahahahah
Donate it to someone who can't afford a computer? That's what I usually do when I get something like that.
DOS is a different beast. If you can run SATA drive in IDE emulation mode it will run even on the newest hardware available, and with SB Live/Audigy/Aureal Vortex, some C-Media cards etc you can even get a sound and music. In fact i successfully launched Extreme Assault natively with 3Dfx Voodoo on Phenom II X4 955 with 32Gb RAM There's no real reason to do it though - DOSBox Daum build supports glide in Windows/Linux/OS X, and there will be less power consumption (DOS can't send idle signal to CPU and it runs with 100% load, i heard about some kind of CPUIdle for DOS but not sure how it will work on multicore CPUs)
Also Windows 2000 still thought threads and cores on a single chip were discrete chips and scheduled threads in a dumb way for cache-misses galore.