You can try adding swap but I doubt it will help much as the disk is so old
and slow. Adding another 128MB or 256MB of memory would probably help the
most with that system, but given that it has a sub 200MHz 486 class
processor, you really need a more modern system if you want decent GUI
performance with modern GUI apps like FireFox, ThunderBird, Opera, etc.
I haven't tried running a full Linux GUI desktop on really old x86 hardware,
but my gut instinct tells me you'd really need at _minimum_ a 200-300Mhz P6
class machine (anything Pentium Pro or later but no cacheless Celerons) with
at least 256MB RAM, preferably 384MB or more. A 200MHz Pentium Pro has
about 4 times the integer throughput and 6 times the floating point
throughput of a 133MHz 486 clone such as the AMD, Cyrix, or TI chips. And a
200MHz PPro isn't going to be super responsive with a modern Linux GUI
desktop either, though it wouldn't be as frustrating as your 486 class system.
If you can, get a newer system. If that's not a possibility, try to get
more memory for this one. Oh, and with only 128MB and no swap, I'd
definitely add some swap, at least 256MB, just to stave off the OOM killer.