Dude, if possible you want to get a card with DX11. Future games will be utilizing this. If you go to newegg.com and search video cards you will find there are many options from under $100 to over $500. You just need to make sure it will fit in your case.
Here is an example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150467 $119.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150448 $144.99
These are both very good options. The 1st one has a minimum requirement of 400w psu and the 2nd needs 450w. You need to check your PSU cables and see if you have any 6 pin that are not in use. If you have one you are good to go.
You can go for an NVIDIA card but they do not have DX11 yet. Older NVIDIA cards like the 250, 260 ,275, 285 or 295 series are all very good cards if you choose that route. In my opinion I would go with a card that offers DX11 as that is what's in the future for games. Also, XFX has the better warranty so keep that in mind when shopping
Good luck!