Most "old" games (aka 16 and 32bit) run on XP, when NOT installed to the "Program Files" Folder.
A very few games need Windows 9x on real hardware, preferably running on Windows 98SE.
For DOS-based games, 98% run on DOSBox, and most definitely run better on DOSBox that on real hardware.
So having a machine Dual-booting W98SE and XP will cover all you need. You'll get hardware-suggestion at your other mail.