Use an online memory configurator to make sure it fits. Not all ECC will work. Kingston and Crucial both have memory conficurators at their homepage.
Most HP/Compaq and IBM servers use memory with specific "security chips" and will not accept just any memory. ECC or not. Also in most cases you cannot mix buffered and unbuffered memory (ie. registered ECC). You must know what you have currently!
Better safe than sorry!!!
-DaFinn