Welcome to Android Central! Yes, they use RAM, but your system should still be able to intelligently manage RAM effectively, so I would suggest not worrying too much about it. I find that most widgets aren't that useful for me, so I tend not to use them anyway. Calendar widgets don't show me enough detail, and social media widgets like those for Facebook and Google+ are not that helpful. I can use Google Now for weather, so no widgets needed there. The only widgets I use are a clock widget on my phone, and a Gmail widget on my tablet.