Personally I would suggest a Dell Latitude D600 for Windows 98 SE, and/or a Dell Latitude D530 for XP. The D600 is probably the newest model computer that supports 98 SE, and the D530 likewise with XP. XP does have better older app compatibility than any Windows newer than it, but it's still a little limited for the super-old DOS games. I got the DOS version of MegaMan X working great in XP, but the original Duke3D doesn't seem as friendly, so it's definitely a little spotty.

For XP, the GoG redistributables aren't quite as good simply installing and running the original copies aside from already having the bonus features. The GoG version of SimCity 2000 for example is a DOS box, when there is a Windows version that runs great with XP which can run in full screen @ native colors and resolution. I also couldn't get the GoG version of Warlords 3 working in XP at all, when the native version seems fine.