The Tavern

What are the absolute lowest spec's Dungeon Crawl would run on? Someone is offering me an ancient P120 mhz laptop for basically nothing, and I figured it might be decent for playing Angband and Dungeon Crawl while my son watches cartoons on the PC upstairs.

With that old of a machine, you might be more pressed to find an OS that Crawl will run under that works on the machine. As far as I know, at least for console the specs are pretty darn low, but the library dependencies mean that you need a relatively (compared to a 120Mhz machine) up to date environment.

we dropped dos/win95 support so yeah, the OS might be an issue. Console should work fine with a light linux distro. Playing online should be quite easy too.

I think it comes with win98 but I'll likely replace that with Debian and the lightest environment i can find.

With luck you could even get a nice console result in a x-less environment on a modern debian. Provided the laptop has a nice resolution, you could get at least 80x50 console which is very useful.
And irc on the other console for the bots

You may need to dig up an older version of Debian if you go that route. Depending on the hardware, you may not have drivers available in the installer for things like the hard drive. (Which can be worked around with putting the drivers on a USB stick, but it's something that we deal with sometimes at my job and we're not allowed to use the flash drive method. Hence my needing to build some PXE installer rigs with all of the nonstandard drivers built in.)

Drivers may or may not be a problem. I've found that the only problem I tend to face with old computers is that none of the current X environments like to cooperate with the hardware, whether it be a problem with the video driver, an incompatibility with the video card, or simply a matter of the card being too damn slow. If it can run Win98, you shouldn't have too much of a problem

I suggest a minimal Debian Squeeze install with an LXDE desktop. If you don't like LXDE, use XFCE. With a 120Mhz computer, this should probably work all right.