Decker doesn't have any per se minimum requirements; on older and slower computers it will run worse, but I'm always trying to make it usable on a wider range of devices. Some time ago I made changes to allow it to be compiled against SDL 1.2, which means that in principle it could be built to work on windows 98. RAM consumption will vary based on the size of the loaded deck and how much undo history you've accumulated; perhaps somewhere between tens of megabytes and a few hundred?
I did most of the early development for Decker in a not particularly beefy 9+ year old macbook air; Web-Decker should be usable on most inexpensive Android tablets or Chromebooks. Two years ago I posted about Decker being usable (if a little bit pokey) on the OLPC XO-4.