Hello, I just purchased an used 21.5 iMac mid 2010 with 4GB of RAM and 500GB hard drive for $150. I just needed something for my mom who's visiting for a month so she can get online. Of course I plan on keeping it because I'm sure my wife will use it. I was wondering if anybody out there got the same iMac model as me and is currently using Sierra on it. Is it running smooth still? should I upgrade to Sierra? or should I stick to what I have right now (Yosemite 10.10.5). Thanks!