Ideally you want matched sizes and speeds. The same brand wouldn't go amiss either but as it's winchip I doubt you'll find those with any ease. They do ODM for other firms and do crop up in some strange places but you'll rarely seem them advertised. I recall Geil use them sometimes and Corsair also do on some ranges of their memory.
Grab Speccy ( http://www.piriform.com/download ) and see what the finer details of the memory are and we'll go from there.
ScanProAudio & 3XS Audio Systems