My friends ask me to build him a web server with 12.04 64-bit server edition in his 1GB RAM server.
Is it recommended or safe? Because I heard 64-bit system need at least 2GB RAM.
The server guide lists the minimum requirement as 128 megabytes for all architectures. I don't see any reason why 1GB wouldn't be fine.
However, that's just the installation. How much RAM you should have to run a particular workload depends on what that workload is.