First of all, the trick for fixing Xorg when it cannot auto detect the correct settings. You make an USB key or a CDROM with Puppy Linux Wary on it, boot Puppy, open the Xorg configuration tool and copy the "xorg.conf" resulting file, disable or remove the not needed parts (there are comments) and copy it in your X11 directory. You may wonder why Xorg doesn't have a configuration utility. The answer is the developers think it is capable of auto detection and when it fails, you must be punished for your sins so you must write the "xorg.conf" file by hand. By the way the "xorg.conf" is deprecated so one of these days I expect the developers to quit supporting even this last escape. If auto detection fails you throw away the PC.
Here the news, what to use with a old computer. Not "old" in the american sense, meaning one of two years old, more like 10-15 years old.
1. the said above Puppy Wary
The problem with it is that the distro is thought as its author's personal toy. It works and it does many tricks but in the same time it is a pile of strange tools organized around strange menus. Maybe you can use it for some real work, I don't know.
It works with old hardware like the above but it looks less "swiss army knife" and much closer to main stream distros. In this case the limitation is software. There is almost everything for common tasks in the repository but not more than this and it is not much updated. I use it as backup on a secondary partition. Beware that "rolling" here means "it doesn't work", go for the "stable" version.
I have just installed it as main distro and it works. It is based on Ubuntu LTS and uses Enlightenment as desktop environment. The reason to use it on old computers is that is requires few hardware resources. The other pros are that it looks good and it allows to install updated software from Ubuntu/Debian repositories. You can also get all the "non-free codecs" to play videos and such. In fact it is the ONLY distro that plays Xvid files on my PC, using VLC. There is a flaw then, it doesn't come with applications by default, you must add them post-install using Synaptic, apt-get or from their Web appcenter. And it can be painfully long. On a side note: in theory the good thing of Enlightenment is that you can set almost every single detail of the desktop environment, all the bells and whistles and make your own theme or hack some existing theme. But it is the same as common GTK themes or icon collections, there are maybe 2 or 3 that look good enough to be really usable, the others either look amateurish or 20 years old or just you must be color blind. Plus you must have time to learn what all those settings do and to test the changes. So I just did what I usually do, I changed the less as possible of the default theme, re-arranging the bar and adding a more colorful wallpaper.
Meanwhile I have quit with:
1. Slackware based distros because they seem to be incompatible with my video card. Otherwise I would recommend Salix.
2. Crunchbang, openbox based and Debian derivative, fails during the installation for some reason.
3. Arch based distros are true rolling, updates every day and each time they can break the system, plus you must install everything or at least part of the software by hand as above. Too extreme for me.
Everything else is just too bloated.