Until earlier today, it was an ancient Pentium 100MHz (f5 m2 s5) with 56MB RAM running Smoothwall GPL 1.0 patch 10.
Now, at last, it's an ancient Pentium 100MHz (f5 m2 s5) with 56MB RAM running Smoothwall Express 2.0 patch 4.
When I have some spare time, I'll replace it with an ancient Pentium 100MHz (f5 m2 s5) with 56MB RAM running a hand-cranked firewall on a 2.6 kernel and something like Turtle. One day. It'll still be a stupidly overpowered machine for the task.
NTL are still being a bit crap, dropping traffic for a few seconds at a time once or twice an hour.