AMD recommends, on their site, the R7 240. However, there are videos of people using the R7 250 in Dual Graphics mode - unsure if that necessarily means it will work better, though. The R7 240 and 250 are the officially supported GPUs for Dual Graphics.
If you kept details settings to medium, you would get acceptable framerates.
If you have the extra money for an R7 250X, just invest in the Athlon 860K. With the extra saving, you could get a R7 260X.
Now is actually a really good time to get the 860K, it's on sale for $12 off:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113379&cm_re=Athlon_860k-_-19-113-379-_-Product
if you planned on getting an r7 250X, by getting the Athlon 860K, you could invest in a R7 265, which would be huge upgrade. You should be able to play most games at 1080p with no issues.