As above, Win 7 is a free upgrade from 8.1. Discussing this yesterday when looking at new hardware from sellers to the professional market, plenty seem to come with 8.1 licence and 7 installed – my understanding is that a lot of businesses are doing new 7 installations and certainly not moving onto 8 (in fact I’m wasting time trying to get my brain to solve problems in the background for a new Win7 VM install I’m busy working on). 8 is very much the unloved child in the same way Vista was. There is a MS server which provides legit Win7 installers as a download if you have an 8.1 licence key.
If you do want to go to 10 though, it’s worth bearing in mind that MS have announced free upgrades from all 7 and 8 licenses, whether or not they are legal 😈
Oh and I agree with the comments about XP x64 – that wasn’t even a good choice when it was the only 64 bit option.
Finally, I’d recommend a move away from XP for all users if you have any connection to the outside world, whether or not you want more memory. Still have some here until the W7 gets rolled out on the second server, but we have a specialist environment which avoids some of the issues, and I still feel uncomfortable with that a year after we should have stopped using it.