I always do a clean install. Write down the programs and just download and reinstall them. Gets rid of all the junk on the pc.
As for steam, yes it will be affected. You need to go onto in steam, right click a game and click back up game files i think it is, then back up all games so some folder of your choice.
After reinstalling steam after getting the new SSD, you can restore all the games straight from the saved backup folder. Beware though, you cant choose individual directory locations for each individual game. So, if your steam games amount to more than 120gb, or around 90gb after windows installs, it wont fit on the SSD, so you may have to put them all on the old hard drive. To do this, while installing steam after getting the new SSD, create a folder on the old hard drive called Programs, or Steam or something, then choose the directory as that to install the Steam program to. Either that or just choose not to restore some of the games as I take it you dont play all your steam games every day.