Arch is pretty light weight, it doesn't install so much stuff like Ubuntu and similar distros. It doesn't install anything that you don't want to. But it is a lot of work to set this up the first time. If you like to have a lot control over your OS, Arch is something for you. If you don't, try something else.
You might also want to check out BSD type systems. FreeBSD doesn't use much resources and is pretty cool.
I used Red Hat when it was free many years ago, then switched to Fedora Core when they made Red Hat Enterprise as commercial software. But recently the Fedora Core has gone the pen-drive way like Ubuntu and such, which I dislike. Not to mention that they are bundling it with Gnome 3, which sucks in my opinion.
That's when I started looking for new distros and I find Debian quite OK for a workstation and started using FreeBSD for a server system.
I suggest using BT (backtrack) 5 for the tools - special pen-test distro loaded up with all sorts of tools. I checked out Blackbuntu, but it was crap compared to BT.
In the end it depends on what you want to use the linux system for - server, workstation or just for linux tools on a bootable media. Of course you can mix and match to some extent as you can use server software on workstations and install pen-test tools on the server machine etc.