Dx10 makes a bit of a difference though its little things that make big differences, when its properly used though it adds that little bit that most games need to take it from really realistisic to "wow that looks totally real".
Its ironic Vista will be great for your average business customer its much easier to use than XP for someone with little knowledge but these peoplle usually dont have systems to run it or not quite good enough although this is mostly down to the ram on laptops most desktops nowadays even standard business ones come with a gig of RAM, laptops however are still standard at 512Mb. Then you get the computing professionals and universities where people know how to use computers and generally have enough powerful systems to run a whole network of Vista PCs they dont really need what Vista brings to the table over XP.
Most of the new FPS games due it are DX10 capable however it is totally backward compatible thankfully so they will work fine with DX9L. Ms were supposed to have redesigned how windows handles games so as to get more performance doing an automated boot to a specially designed no GUI version(like we did with DOS in the old days but automated) and run the games from the cut down low memory mode then auto reboot back to normal windows however i believe they dropped the feature, shame its really what vista needs.