xp does not have (a non hacky faked) 64bit support.
thus modern games are starting to require a stuffload more ram then a 32bit OS even can use.
thus modern video cards who's dx10 and 11 ONLY works on vista/7 and 64bit OS to boot.
yes, xp users shouldnt even call themselves gamers.
its not a new age that just hit us, this is something thats been happening more and more during the last 5 years.