I can't remmeber the last time my PC only had 1 HDD. My OS is always on one (as well as apps and secondary things like vent/fraps) while my games, pictures/movies etc are stored on the second/third HDD... that way if windows or the OS HDD gets FUBAR'd I wont loose my unreplaceables.
And its simple to make work? You simply see all your drives in My Computer?
If you ever go for a pre-built you have to look for the ones which have been either test PCs or Window PCs [the ones which display powerpoint for a while] which is what I've got, they are the only cheap ones.