No, you will not have to re-install. Just turn off your computer (completely - no hibernation!) take off the side-panel, and insert the RAM. Once you have done that, turn the computer back on. Once you have established the RAM is working correctly. (Use memtest if you like, although you will have to boot that off a usb), slide the side-panel back on, and you are in business.
Here is a link for memtest if you are interested - I definitely reccommend it:
http://www.memtest.org