from the sound of it, your entire system is rather outdated: 2gb of ddr2 667Mhz ram would imply at best an older socket 775 or AM2. Buying a video card thats high end might be a waste due to CPU limitations. also, does your motherboard even have PCI-express 2.0 slots? The new cards are backwards compatible in the older slots, but it would limit the card if this were the case.
While CPU limitation is usually minimal, knowing whether you have PCIE-2.0 or not is important because that directly affects your graphics card's bandwidth. Anywho, Depending on your computer's specs, I would recommend either a Sapphire 5770 or 5850.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102868http://www.newegg.com/Product/Product.aspx?Item=N82E16814102857