The oldest computer I have, that will run a modern Linux distro, with beautiful graphics, et cetera, is an eMachine eTower (circa 1998 / WinME). A friend donated it to me -- 667MHz Celeron, 192MB RAM, Intel GPU, blah, blah, blah. Soooo, it is doable, but...
The only distro I could find, that would boot from the HD (and, I tried many), was Macpup Opera 2.0 (a Puppy Linux fork). The eMachine was an extreme example of mostly proprietary hardware, but Puppy Linux et al. works just fine, generally speaking, on most ancient iron.
I also have a Celeron-powered netbook, bootable USB sticks, and so forth that require a light OS (otherwise they are intolerably slow). On modern mobile devices (laptop, netbook, USB sticks) I run Peppermint OS (an Ubuntu fork). It flies like the wind, on modest hardware.
If it was me, I would be looking for a modern Linux distro, that was designed around an older kernel (intentionally older, for older hardware). That's what I recommend, and what I do.
As time goes on, older hardware support starts getting pulled from the latest Linux kernel(s). They do that mostly to keep the size down. Kernels are constantly changing -- support for newer devices is added, and support for older devices is removed. P3/i386 support has totally been removed on the most recent kernels. I'm running Linux 3.10 successfully on this P4/i686 box, but my time is coming, too. The distros I mentioned above (and personally run myself) will continue to support i386 machines, via older kernels, for some time to come. It's their 'bag', you know?
Anyway, I would burn a couple of bootable CD's with Macpup Opera 2.0, and Peppermint OS -- boot them up, see how they run (I think you'll be amazed at the difference in performance). If everything goes well, install one_or_the_other on your HD, and call it a day.
Otherwise, you might get away with downgrading your current OS, and once you get it working again, start pinning packages to protect them from being upgraded in the future.