Postgres scales great up to 256gb, at least with 9.4. After that it'll use it, but there's no real benefit. I don't know about MySQL. SQL Server scales linearly with memory even up to and past the 1TB point. I did encounter some NUMA node spanning speed issues, but numactl tuning fixed that.

I setup a handful of pgsql and Windows servers around this size. SQL Server at the time scaled better with memory. Pgsql never really got faster after a certain point, but with a lot of cores it handled tons of connections gracefully.