You could use Debian x32 instead. 32 bit pointers but amd64 instruction set: https://wiki.debian.org/X32Port
If by that point you can't just throw another 512 of RAM at the problem, I'd be worried. 1GB is already starting to become the lowest rung for VPS providers. That's the $5 plan at Linode, anyway. I fully expect that to be universal (or greater) in four years time.
And that's not to mention that while a little wider, 64bit is also faster. This could be a saving factor under load.
None, the default install of Arch leaves you with a terminal prompt and basic utilities. You can install whatever you like from there. It's closer to something like the Debian minimal install than Ubuntu desktop.
On a related note, Unity is getting the axe and 17.10 will ship GNOME as the default DE.