So I've been messing with Arch on my old Compaq Deskpro EN with a Pentium 3 733MHz, 256 SDRAM, Voodoo3 2000 PCI 16Mb, and a 40GB hard drive. I must say it is a very responsive and usable system. It runs firefox with out any problems and even youtube with flash, as long as I keep flash videos their default size it runs them smoothly like any modern computer even most high definition content on youtube runs perfect.
Here is htop, very little ram is being used. Despite the fact that I have synergy, nitrogen, openbox, lxpanel, conky, terminal & X running.
Here is a video from youtube in HD, running perfectly fine. Ofcourse fullscreen is beyond what this system can do with flash. But it can play regular divx and xvid videos with no problem, fullscreen or not.
29 seconds. The default boot on Arch, no optimization.
25 seconds. I saved 4 seconds, by simply loading all the modules in the background. The system was completely functional by the time X started anyways, 2 seconds later.
20 seconds. I recompiled the kernel, by using this kernel. Basically removing everything not needed and setting the kernel specifically for a Pentium 3. I did not spend too much time going over each option, so there is A LOT more optimization I can with the kernel configuration.
Here is the .config file I used for the custom kernel in case anyone wants to reference it.