Hi, hope someone can help. I have a HP Z400 workstation - 4 Dimm slots, currently using 2x2G ram. I'd like to upgrade to 8G and I'm confused by RAM compatibility. According to HP manual I require ECC Unbuffered RAM. That's easy enough. I've looked up Kingston and this is the list of compatible RAM they list for my workstation. http://www.kingston.com/us/memory/search?DeviceType=1&Mfr=HEW&Line=Workstation&Model=55069 I've found a site in the UK that sells a compatible single 4G stick for 31 pounds. http://www.ebuyer.com/244940-kingston-memory-4-gb-dimm-240-pin-ddr3-kth-pl313e-4g I've also found this set of 2 x 4G Kingston Ram but it is cheaper. The only thing is, this set was not on the compatible list and is a different part number. http://www.newegg.com/Product/Product.aspx?Item=N82E16820139262 However, specwise, I don't see any difference other than the more expensive one is system specific and the cheaper one has thermal sensor. They are both ECC unbuffered DDR 3 1333 (PC3 10600) Ram, CL9 1.5V. Is this RAM compatible with my workstation? Or should I stick with what is recommended?