You have a 1.9mhz dual core processor of an old architecture. It is going to limit either of the HD5770 or GTX 460 a large amount, especially at a low resolution like yours. If you don't want to upgrade the processor this HD4830 is just $60;http://www.newegg.com/Product/Product.aspx?Item=N82E16814131381
It is a good deal and you really won't see much benefit from a more powerful/expensive card, if any.
If you are willing to upgrade your processor Asus does say your board can handle AM3 processors;http://www.asus.com/product.aspx?P_ID=szSFtrap7crpBaQE
You'll want at least a dual core at 3ghz or higher for current games but if you can afford it a Phenom II x4 is the best option. After upgrading then the cards linked in your best answer are good choices.