This is what you want for $150 or under right now.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102932
Great deal but that said shadow is right about the processor limiting cards in that price area. Your resolution is also pretty low so that card is kind of overkill.
You may just want to save some money and get something cheaper and more appropriate for your current CPU/monitor. This GTS 250 is also a good deal and probably your best choice for your current setup;http://www.newegg.com/Product/Product.aspx?Item=N82E16814150513
It is about 50% faster on average than your HD3870 at 1280x1024.