If I am understanding your description you have an AGP slot? Your better cards are PCIe x 16 and AGP, although, the AGP cards are becoming fewer in numbers. Anyway, the things to consider when purchasing a graphics card are your systems requirements. Most of your lower end cards do not require extra power to operate. You can spend alot of money trying to gain the perfect system. I personally like the ATI/AMD cards becaused you get your graphics and, you can also get the HD sound out of them. You more than likely have a 250 or 300w power supply stock with your system so you would want to stay in that range of card unless you want to replace your power supply.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161318
Something like this would work for you if youhave a 300w supply. You can browse through and look at your options.

Something like
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=6970133&CatId=318