Requirements difference between Yosemite and Sierra

I have a late-2009 iMac which seems to have held up pretty well, especially since upgrading the RAM to 12GB. I only fairly recently updated the OS to **Yosemite** after running Lion for so many years. The OS update my have made my Mac a *little* slower, but if so then not by a great deal. But I could probably do with updating again — to Sierra — as certain programs require a more recent OS than what I have. I am a bit concerned that running **Sierra** will put too much of a strain on my machine... or **are the requirements not a whole lot different to any other versions of OS X over the last 5 years or so?**