Thank your brother, he gave you good parts.
The 850w power supply can run any single graphics card made today.
At the $380 budget point, you are looking at a GTX570, or 6970. Either will fit your motherboard and psu. They are both comparable in performance. Either will drive a single monitor very well in games.
Here is a EVGA GTX570 superclock:http://www.newegg.com/Product/Product.aspx?Item=N82E16814130595
or a comparable card, the sapphire 6970:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102957
I tend to like EVGA for their support which should be rately needed.