Gigabyte GA-586ATX3:
Intel TX chipset, ATX form-factor, SD-RAM, UDMA-33, but limited to 66MHz FSB though. And no, clock gen doesn't have any undocumented settings. It is 60 or 66 MHz. No more, no less.
Put in 64MB RAM, an ATI Rage Pro Turbo PCI with 8 Megs (and maybe a Voodoo 1), flash the pimped BIOS for 120GB HDDs and let it run.
Another option: Shuttle HOT-599. Although it is SS7 which could be a bit overkill for a P233MMX it is a nice & compact board. µATX (I like µATX 😉 ), 3x SD-RAM, VGA onboard, Yamaha sound chip onboard, FSB options up to and above 100 MHz. P233MMX @2,5x100. Could be interesting.
RetroPC: K6-III+/400ATZ @6x83@1.7V / CT-5SIM / 2x 64M SDR / 40G HDD / RIVA TNT / V2 SLI / CT4520
ModernPC: Phenom II 910e @ 3GHz / ALiveDual-eSATA2 / 4x 2GB DDR-II / 512G SSD / 750G HDD / RX470