You'd want to get ram that has identical specifications (other than size). When you mix RAM with different specs your PC resorts to running both at the lower specifications and so you lose some performance.

This person reported that their alpha came with "Samsung M471B51733EB0-YKO, 4GB PC3L-1600, 1.35V, CAS 11". Since the stock memory is CAS 11 you could go with something like this: "http://www.amazon.com/gp/product/B00CQ35HBQ/ref=oh_aui_detailpage_o09_s01?ie=UTF8&psc=1" that's competitively priced from a relatively well known manufacturer.

Lets say you wanted to shop around on amazon. You'd be looking for 8GB PC3L-1600, CAS 11 and 1.35v.

I bought this ram for mine and it works fine https://www.amazon.com/dp/B00CQ35HBQ?ref=ppx_pop_mob_ap_share