Ooops. 48GB here, regretting that I didn't get 64GB. (My excuse is that I needed it for osm2pgrouting which ate up to 90GB of paged memory on a country-sized input file. That hurt a lot.)
I have never been gladder that I abandoned my OSM data-processing project before I got that far. (I was planning on processing multiple country-sized input files for their road networks – on a friend's computer, but it only had 16GB, so that would be a lot of paging.)
I then jumped ship to OSRM, which did the same job in ~3GB of memory, AND several times faster, AND didn't return spurious routing results afterwards. (I was getting some routes with "hyperspace jumps" between points in the road network that weren't connected, and due to previous issues with memory I just decided to stop trying, so I didn't investigate the cause of those jumps.) So if you'll ever decide to do that again, just use OSRM. It "just works" (at least in my experience).
