The only thing I'd like to know is, are you on a tight budget? And what games would you like to play in what settings. Because upgrading an AGP card these days is bound to have you searching all over the place, since these cards are hard to find. You might consider upgrading your whole PC, while your at it. But that solely depends on your budget.
If your budget only has room for a new graphics card you might consider getting a 3850 or so, I believe these come in AGP form.
Here it is:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102730