Definitely XP over Vista, although i would choose x64 edition, it's much more stable, and is faster when playing with bigger files.
Also, go for a Quadro over the 8800. They cost more but are a lot more reliable, especially when it comes to ArchiCAD. Check the compatability database on ArchiCADWiki for which model to pick.
Josh Osborne - Central Innovation
HP Zbook Studio G4 - Windows 10 Pro, Intel i7 7820HQ, 32Gb RAM, Quadro M1200