in normal cases you have to turn of the computer or workstation and change the ram, in some cases you have to make an bios update, but only if you want to go from 16 GB DDR-3 RAM to 24 GB DDR-3 ECC registered RAM.
And you should also use a 64 Bit OS regarding to the 4 GB limitation at the 32 Bit systems.
OS and software need not to reinstalled, but there fore if also have seen some rarely cases the owners needs to do so,
so the best tip I can give you is even before changing any hardware to do a backup. If you are using Windows 64 Bit
Pro, Ultimate or enterprise it should be done with the new Windows backup imager, if you are using the ultimate you are able to backup the data over the network if you want and need. if then something occurs, like a fail you are able to reinstall the windows system and also the backup, so if the computer is able to use in the most cases within half a hour or one hour later normaly likes before.