Adding RAM won't hurt any of your existing files, settings, programs, or anything. Just pop open the case to see how many DIMMs (memory modules) your motherboard can accept. On some older computers you used to have to add RAM in pairs or put certain ones in certain slots. I don't think that's the case for anything these days, thankfully...
If you add RAM the only thing you're likely to have to do is enter the BIOS once the first time you boot it up, exit and save everything, reboot. Good to go from then on.
