I'm not sure how CPU dependent GW2 is, but you'd be looking at a 6850 or 6870. Important to note is that you may need a new power supply. Do you have the model of your power supply or computer if you bought a pre-made?
Those are my current PC specs. I want to play this game: Guild Wars 2 minimum requirements on high settings if possible. What do I need to upgrade, and what changes can I make between $100-$200 to run this game on high (or medium if that is all I can hope for)?
Is a new graphics card all I need? And if so, can anyone recommend me a card?
6870 is the better card as its more frames for about the same cost, but a 6850 will do quite well. A GTX460 will also be a good choice if you prefer nVidia.
460 - http://www.newegg.com/Product/Product.aspx?Item=N82E16814127646
6850 - http://www.newegg.com/Product/Product.aspx?Item=N82E16814161384
6870 - http://www.newegg.com/Product/Product.aspx?Item=N82E16814102948