Although COD: Ghosts won't be released until +5 months from now, the game has now been made available for pre-order on Steam. There are no pre-order bonuses, for now, but they will surely be announced later this year.
"Fueling this boldly reimagined Call of Duty universe, the franchise's ambitious new next-gen engine delivers stunning levels of immersion and performance all while maintaining the speed and fluidity of 60 frames-per-second across all platforms."
The page states that the minimum system requirements are still being determined. Although DirectX 11 is listed, which makes sense since Ghosts uses a next-gen engine, it's likely that a DirectX 10 graphics card should suffice. Users are most likely required to have Windows Vista or higher since Windows XP doesn't support DirectX 10.