Strategy
PC strategy games are a game truly perfected by a mouse controls, a frachise totally uncomparable by console inferior designs, (take the example of starcraft for the Nintendo 64). Although only a handful of programmers have done the genre justice, these few games can have a lifespan for several years or even up to a decade in the case of starcraft and warcraft series. The Real Time Strategy or (RTS) gameplay consist of armies, races or whatever you want to call them being taken in command by a god-like figure (yourself), which you will guide to victory on a path of strategic analysis, artistic gameplay, or mindless pillaging.
Graphically it can be said that the simpler the better, to get a great understanding of the battlefield sometimes less detail is much easier to utilize when it comes to navigating troops. Something that truly shines on a good RTS is complicated gameplay but an easy understanding of battlefield views and sighting of a specific unit. Luckily, this will make your graphics card purchase a little cheaper, although you never want to go too far below video card requirements because laggy gameplay can be devastating on the battlefield.
If you are going to be playing a game as old as Starcraft or the older Warcraft you shouldn't have the slightest problem with running it, with a simple requirement of 16MB of Ram and a request of directX I don't think your computer will have any disputes unless your running it on a caculator.

Lets take a look at some of the newer titles though: World in Conflict a great new looking RTS which has you pick your class of units such as Air, Armor, Infantry, and support which adds a unique class system to online play. This is one of the better looking RTS games and has a very intuitive camera changing system, which will let you view your territory at almost any conceivable angle. This game will only require 1 gig of ram and a 128 Mb video card, and doesn't make the jump to directx9 so it doesn't have to have Vista.
Another realistic war strategy game is Company of Heroes, a World War II RTS that is pretty robust in it's single player and has great balanced units system. The game almost seems like a mix of modern and 1940's weapons, and overall the game kicks! I recommend 1 GB of Ram and 128 Mb video card, (the same as World in Conflict). Although, I have seen that the maximum requirements can go as high as a graphics card of 256MB, which I have not personally tried using that powerful of a Graphics card, and I find that 128MB works just fine. A few other games worthy to talk about that will require the same amount of graphic power and therfore video card are: Command and Conquer 3: Tiberium Wars, Civilization IV, Supreme Commander
It wouldn't be a genre if we didn't talk about Blizzard's two cents, since over a decade ago the company blizzard has revolutionized the way we play RTS' on the computer. Games like Warcraft and Starcraft were the most balanced, best designed, and greatest gameplay out of any other real time strategy. Not to mention that for its time Blizzards cutscenes, in games like Starcraft, were some of the best ever seen. The new Starcraft is coming out, and I suggest you
check it out!.