I think you could go either way but in all honesty I would go 2 computers over a dual boot.
your going to run into software and compatibility issues with the old stuff, some of the turn of the century stuff has a problem recognizing a "compatible render device" in a newer computer.

plus with the hassle it would almost be worth it to scavenge really SUPER CHEAP parts for your old school machine and then build a halfway decent modern machine
I think for your old school machine you could probably get away with something with DDR2 and a 1st gen pci express video card. that combo puts it pretty squarely in XP compatible era albeit near the end.

just remember to get a KVM so your not having to swap cables around all of the time