If your resolution is below 1680x1050 then this is an excellent deal;http://www.newegg.com/Product/Product.aspx?Item=N82E16814127452
It should perform very well and will be ok for your 420w PSU. Like almost all decent card it does need a 6 pin PCIE power connector however. Your PSU may or may not have one and if not you will need an adapter as from the pics I don't believe one is included.http://www.newegg.com/Product/Product.aspx?Item=N82E16812887001
As for Nvidia vs ATI which is better depends on what cards you are comparing but in general right now Nvidia's card over $100 are overpriced for their performance compared to their ATI counterparts.