This is a fun and nice thread, I must say. I'm one of the few nutjobs here (most likely the only one) who actually uses a retro machine daily not just for gaming, but for general use. The computer in my signature is my main computer.
Insane? Yeah. Stupid? Of course. Overkill? Definitely. Stupidly overkill? Absolutely.
What I mostly find fun is to try to figure out the limitations of the machine itself and trying to overcome and get around them. So far, I have succeded, since it runs Windows XP and browses the web at a very good pace. Using this overclocked Pentium III Tualatin as my daily driver for all my daily tasks also taught me how to optimize the machine as much as possible and get the most out of it. If it wasn't for this system, I would have never developed the UOC Patch and the Enforcer, for example.
Right now, I'm waiting for three new RAM modules, that should let me push the FSB to 150MHz (hopefully, considering that the chips of those sticks are the ones recommended for overclocking at 2-2-2-5...), a 128GB SSD in order to get rid of mechanical hard drives for good, and hopefully, an SL6BY Tualatin, which seems to be suited better for overclocking.
My Retro Daily Driver: Pentium !!!-S 1.7GHz | 3GB PC166 ECC SDRAM | Geforce 6800 Ultra 256MB | 128GB Lite-On SSD + 500GB WD Blue SSD | ESS Allegro PCI | Windows XP Professional SP3