Lucky_SLS :
]u sure the op doesn't want to upgrade to 16 gigs later?[/b] a 1x8gb ripjaws 2800 is what i would recommend. it can be overclocked to 2933 easily in that b350 mobo. op can add another stick if he wants later. lol,
"Adding" RAM can be problematic, and dual-channel ha performance gains.. Regardless of what configuration used to start with, I'd always recommend replacing with a new "kit" (so in this case, replace 2x4GB with 2x8GB), but the additional may simply work.
Personally I'd opt for the 1400 + 8GB 580, but you've given a solid alternative with the 1500X + 4GB 580.