I was going to find a better place to post this, but perhaps this will help:
I first installed a DTC PCI SCSI controller to run an old IBM 1gig SCSI-II drive. Figured it was cheap storage. I installed 98 with no problem, rebooted at least a dozen times. Installed video, sound, and USB drivers. Walked away. Came back two hours later to copy over the USB Zip drive drivers and the thing wouldn't start - it would lock up hard during the Win98 startup screen. Booted into Safe Mode, disabled all the drive-related items under System Properties->Performance. Rebooted. Got into the GUI, then it locked.
Today I picked up a TekRam UW PCI SCSI controller, put it in the WebSurfer. During the SCSI BIOS screen, the TekRam reported the error "IRQ 10 assigned, unable to access IRQ, BIOS disabled." Hmm... Went into the system BIOS, set IRQ10 to ISA/Legacy and rebooted... same error message on IRQ9. Back into system BIOS, disabled all high IRQs (9,10,11,12) and rebooted. This time the TekRam was assigned to IRQ 3, no error message, and the system runs great.
I have only two theories: One, to save costs the MediaGX chipset doesn't have a "real" cascaded interrupt controller, so it can only assign low IRQs (0-7) to the PCI or ISA bus. Two, the MediaGX assigns its devices to the second interrupt controller (8-15) and doesn't like to share. Either way, forcing the PCI devices to use low interrupts solves all bus-related problems I've had, including installing PCI ethernet.