Always go for more memory rather than faster memory (unless you're some kind of gamer/enthusiast who's going to do some overclocking).
So in your case there's no hesitation in recommending the 2 x 2GB DDR2 PC2-6400 option. G
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.