For HP 4GB sticks there are Elpida black, Nanya black, Samsung silver, Micron blue, and Hynix silver... the color refers to the heat spreaders on the outside. The HP part numbers are 398708-061, 398708-561, and later 466463-061. I like to get a matched set of 4. I've had a bit of bad luck with the Micron blue. I've liked the Samsung and especially the Elpida and the Nanya for these.
Since there only are 4 memory slots for the xw6400 if you're going to soup one of these up these days I'd go for 16GB by 4x4GB. HP sticks are generally "binned" for higher than baseline performance for each stick. That is why I'm sticking with HP memory with their hologram sticker on it.