Windows 7 has a bigger RAM footprint than XP, but once you get over that hurdle, both are nearly identical as long as they are patched with the latest patches.
So if your XP computer can't run Windows 7, it's most likely that a simple RAM upgrade (usually the cheapest upgrade) will get 7 up and running.
I'm running 8.1 via Bootcamp on my super duper Mac.
I really don't see anything different from 7 except it's choking with some of my games, and the interface is more annoying.
The Windows store is ok, I'm liking this Hydro Thunder Hurricane game I purchased thru it. I'm burnt out on "App Stores" though.
Windows 7 has some profound under-the-hood changes including totally overhauled memory and driver subsystems. Memory is used more efficiently and drivers have much, much
less potential to cause full system crashes (due to moving most driver functions into userland.)