I would be disinclined to install Xubuntu
plus Fluxbox.
A Fluxbuntu installation process goes through much the same as a command line install of Ubuntu. It gives you a lightweight system which would probably be okay on the computer you describe. It would probably be worth a try. I have had it on a multi-boot system with no problems.
If you want to do a tailor-made light install, I would recommend this post:
https://help.ubuntu.com/community/In...wMemorySystems
Hope this helps.
Cheers, Pat.