I currently have one 
512MB DDR333 stick on my mobo that supports dual channel. 
I'm thinking of adding a 
256MB DDR333 stick but will that cause any problems?
One more thing, I noticed that there seems to be different models of the same memory. 
I've seen model numbers (from Kingston) as 
KVR333X64C25/512 and also  
KGT KVR333512RI was wondering if it made a difference which one I bought (if there really is a difference at all) 
