Build a file server with the Raspberry Pi

We’ll be using Arch Linux as the operating system for our file server, because it is small,and has only the minimum packages required for a working system. This means that we can set up the file server without wasting resources on a graphical user interface and other unnecessary packages. Arch Linux comes with hardly any RAM allocated to the GPU by default, which is exactly how we want it for use as a headless server.

http://www.linuxuser.co.uk/features/build-a-file-server-with-the-raspberry-pi

1) The official Debian also won't boot to X by default.
2) Fewer installed packages means less sdcard used nothing to do with RAM.
3) Changing the RAM split is dead simple (cp /boot/arm240_start.elf /boot/start.elf)
4) Just a file server (smb) will only use 1% of your RAM so you don't have to worry about memory.
5) As long as you use armhf (http://elinux.org/RPi_Distributions) USB speed will be the limiting factor.
There may be a good reason for you to use Arch but you are not listing it, or any at all.