Blizzard has put so much time and money into wow, as have i...and in doing so they coded that game like the coding gods they are. it is so optimized for all types of systems i am not sure if there is a more compatible game in existence. I have been playing wow since release over 3 years ago and since then the graphics have not changed a whole lot, but you have more options to make them better if you have a nice gpu. as far as minimum requirements go almost any computer running you at $200 bucks second hand, or maybe even free will run wow at minimum-medium grafix.
Funny story is when i first started playing it was on a friends computer, and at the time i did not have one of my own. so i built one out of his spare parts and ran wow like this... 512mb ram, 64mb Nvidia, 800 Mghz (p3 or p2 can't remember) processor and played like that for a year. Your card will cruise through wow at max settings minus AA/Af. Because my old rig had a 6600gt and it did the same.