change the ram to g.skill ripjaw series 8gb 1600Mhz. $47
change the dvd drive to the asus 24x(not the black) $18
Your Psu isn't the greatest. This antec 550w will be better for only $12 more.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016
Don't worry about getting the 6950. He can crossfire later if he wants to upgrade.
The 6870 is a good Video card and fits perfect in a $650 build. If you went with a 6950 you'd probally have to scale all the components down and then you'd have a mismatched pc lol.
Good luck with the build!