Using old hardware to make a torrent box.

I build a small wiring closet in an empty closet in my house with my modem and router. I plan to also put a small machine to download torrents for me to.

The specs will most likely be crap I have in my basement:
Pentium III ~450Mhz and matching ASUS motherboard. Probably very little RAM, assume a single stick of 128mb. Generic video card, not even sure if it will be AGP. I'm probably going to have to buy a small harddrive around 20-40Gb. The machine won't have much cooling since it'll need to be quiet.

The features I'd like are webserver and torrent client+web interface. I want the most lightweight OS and software.

What are you suggestings?

I had a Debian3r0 machine with a K6-2/300 processor, 64MB of RAM, and a 40GB + 8GB drives (the 8GB being the downloading area). It was a headless box (ssh into the machine from a laptop to run screen + multiple btlaunchmanycurses) I used to jack into the University network and download at ~4MB/sec for most of the evening during the Anime club screenings.
The only real performance engineering was running some of the torrents to the 8GB drive (on the second IDE controller) when the torrent hammering (@ 4MB/s...) got a bit much.
I'd expect a machine of half the specs of the K6-2 will be able to handle a broadband connection with ease..