Intel's 440 ZX chipset is closely related to the 440 BX chipset, like Rainmain. It was originally intended to be a low-cost version of the BX that was only for use in Celeron based boards, and as such only allows 4 bus mastering PCI slots and only officially supports a 66MHz FSB. The limited FSB support is accomplished through the removal (or limited availability) of the clock multipliers used to generate the PCI & AGP clocks, although I'm not entirely certain of which multipliers exist.
One of the interesting offshoots of the lack of multiplier settings occurs when overclocking a computer, in that if the FSB is raised from 66MHz to 100MHz, the next "perfect" FSB speed, the PCI bus maintains the proper 33MHz clock, but the AGP port magically finds itself operating at a 100MHz clock rather than the proper speed of 100MHz. I guess some video cards might not respond well to that, but my lowly geforce2 MX doesn't mind at all.