Well for the most part; Linux and BSD, and really, ANY Unix based OS, are gonna have pretty good system support when it comes to running on older hardware.

I mean, any of those things you choose to use, are probably gonna work just fine.

There are some that have more tools than others, and as Coffee Cup / Hybrid pointed out, there's quite a few.

I've personally used Backtrack multiple times. Also, some are geared in different directions; Like some may be used mainly for Forensic stuff, and some might be used more for Penetration Testing.

The one I linked to, BackBox, seems to have a decent amount of tools in general.

I do feel I should at least point out that any Linux distro, has the chance to do this.

There used to be a distro called "I-Slack" but they renamed their project NetSec L, which is basically Slackware Linux, but with a Security minded Package Selection.

I don't even know if it's still around or not, but look into that too. I used to install it on machines here all the time. It's one of th few that came with not only Hping, but Hydra as well.

Hping and Hydra, along with IPSorcery, are some of the better tools out there.

I'll also point out that FreeBSD, has a LOT of Security Tools. Both FreeBSD Ports, and FreeBSD Packages, have multiple Security tools you can use. FreeBSD has a BUNCH of Security Tools available to it.