I think a soundcard takes an INsignificant amount of process away from the CPU. it's used more so for a good surround sound speaker system. not many think "get a soundcard" when trying to improve gaming performance.
definately put aside some money for pedals and the trackir system(if you are sure this is the game for you). maybe look at some HOTAS controls. i use a saitek cyborg x and it has plenty of buttons where i dont have to move my hands around during combat.
i'm running an nvidia 8800 GT and the game runs well, but when i upgrade it's probably going to be a lower end on the new series. maybe this one
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130372vista is a good os and runs well. a lot better than any xp i've ever owned. i've run vista for about a year now and it hasn't failed me yet. it does come with a lot of automatic features turned on as well as Account User Control(which is what most people hate about Vista), but you just need to turn it off.
look up some local computer stores(non franchise). they can set you up with an inexpensive gaming system. building gaming systems is usually their specialty and they should put it together no charge.
and if this computer is truely just for AH2 you shouldn't have to spend more than $500-600 for the pc alone.