mindblast: I don't understand the point. Ok, let's say that some games run better on XP. Oh well, you can dual-boot XP + Windows 7, so i don't see why you would buy a new PC and not use the current one.
You don't necessarily find XP drivers for your new PC hardware, though. Also, some games may have graphical issues on modern GPUs, e.g. because the driver or even the hardware doesn't support some legacy feature that the game depends on.
I have (at least) two older PCs that run Windows XP, and even Windows 98SE. Their only reason for existence is to be able to play those Windows games that have problems on newer hardware, and/or newer Windows versions.
For DOS, DOSBox is enough for me. In fact I think it is better for gaming than a real MS-DOS retro-PC because you can run it at different CPU speeds easily etc. So with DOSBox you have several different MS-DOS PCs in one. Back when I had one old Pentium machine as my dedicated MS-DOS retrogaming machine, I had much better success getting my old DOS games running in DOSBox, than in that real MS-DOS PC. Go figure. At that point I decided to scrap that PC, why should I keep it around if DOSBox does a better job at running DOS games?
I have a few games that don't run passably on any of my PCs, though. For instance,
King Kong. It is the retail version, and its copy protection doesn't apparently work in Windows 7 or later (maybe not even on Vista), so its target OS is Windows XP, and that's it.
However, since it is a 2005 game, I don't think my XP-running Windows PCs have enough power to run it (I haven't tried it yet, but most probably don't). So there I am, I should buy a new 2005-era Windows XP PC, in order to play that damn game. Oh well, at least I got it for free as a gift...
Gamersgate used to have that game DRM-free though and it didn't have at least the copy protection compatibility problem, but unfortunately it seems Gamersgate has pulled that game away from its catalog. Also some reviews suggested that even the GG version might have some other issues on modern PCs and/or Windowses.