i think you might be best to use what you already have, as mixing could affect the efficiency of the better brand if you purchased that. i am not sure of this so please listen to some of the more experienced members comments.
are you experiencing problems with your value ram, or does it seem to be slowing down your system?
if not, i wouldnt change. i have 2 gig of value select corsair ram and run games ect very quickly and smoothly. of course if you buy ram with a better latency it will perform better, but for the kind of things i do on my p.c it dont make a lot of difference.