After looking up your rig it apears that you have a mini tower which you would have to get a low profile card. It is very limited on how big your card can be. The biggest card would be a 6670 by AMD.
Anything bigger you would need a full size case something like this H-14" W-7" L-16". But then again you would be limited in the fact of your AMD 64 dual core CPU.
I will link you to the card that will work in your rig. Good luck to you.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161397