An SSD is one of the best upgrades to make an old computer feel new, and Windows 7, 8, and especially 10, are built to take advantage of what an SSD can do.
Even with an older processor, this author's computer starts up in 15 seconds with Windows 10 running on an SSD.
Even if your files are protected with a secure backup service, you don't want the hassle of dealing with a dead hard drive if you can help it.
RAM makes your computer snappier when you're re-opening or switching between several programs.
It isn't that expensive ($50 or less for 4GB), and it's fairly easy to install.
That being said, after years of increasing bloat, Microsoft made the bold decision to have its new operating systems be MORE efficient.
That's mainly because it was aiming Windows more heavily at the mobile market and weaker mobile hardware, but it works in your favor for desktops and laptops as well.
I have a mid-range Windows 7 desktop computer that I bought five years ago.