Back when it was current, I tried RedHat 6.whatever on a K6-3. I couldn't really do anything with it, and the GUI had an annoying habit of not even running programs. It would show the "loading" animation for the mouse and then nothing would actually happen. All these years later, I've even seen that on a modern Mint install sometimes.
I used Redhat 7.2 and later 7.3 on a Pentium Pro machine, but I was just using it as a basic web server, not a desktop PC. I liked it for that role. This is when I learned ipchains, which is unfortunate because they didn't stick with it. I got confused trying to learn iptables and for years, I would revert back to ipchains instead because it did what I needed it to do. Only recently did I finally start to get comfortable with iptables.
That machine was unable to handle later releases based on the 2.6 kernel. It ran but RAM usage was too high.
The only old linux I used very much as a desktop PC was Fedora Core 2 running on a P2 Xeon. I think that used Gnome 2. Back in those days, nVidia driver support was a somewhat manual process which required downloading something from nVidia, then running some sort of semi-compile process. I don't think it was a full compilation of anything, I'm not sure what it was doing. They also had a serious breakage with the nVidia driver when the linux kernel changed the size of some stack or whatever it was. In the end it was fixed and that machine worked out. Too bad Fedora had such a short support life, and I wasn't in the mood to upgrade it after that. However, I'm not the type to abandon an OS just because it's "unsupported", so I don't remember what the real reason was that I stopped using it. Maybe something else broke.
Other than that flirtation with Fedora 2, I generally did not like using Linux as a desktop GUI environment. I've never liked any of it's GUIs as well as Win2k/unthemed XP.
Recently, I've become at least somewhat comfortable with XFCE on Mint. XFCE is a lightweight GUI but I don't know if it's feasible for a Pentium.
I've had loads of frustration about user friendly configurability of linux distros. The best solution for that I've seen was in the 'yast' utility on SuSE. It has a text version so it was probably there even on much older releases.
I've had a pretty negative impression of linux support for even remotely anachronistic video hardware. It's not nearly as forgiving with this as the Windows world. Older video cards on newer linux get broken and never fixed. I doubt newer video cards on old linux will ever work either. I'm sure they have excuses for why this happens, maybe even good excuses, but it's a problem nonetheless.
Old linux with an old PC is probably a good idea from a hardware support point of view, besides it probably performs much better that way as well.
