Yes, Rampage II Extreme uses X58 chipset 😀
No BIOS mods or BIOS hacks needed (I'm using latest official release).
However, my board has few legacy options build-in (which can be seen on my first video, @6:40).
Don't know how crucial they are for Win98 to work, but other boards may not have them.
From hardware point of view, you need a PS/2 keyboard (just in case USB one doesn't work in installer), a floppy drive (I used GOTEK emulator), and 512MB RAM stick(s). Last one isn't needed if you know how to hack Win98 installer (LINK, I suggest you modify installer for himemx only 😉).
I didn't checked if 1GB of RAM works for installation on X58 platform.
Keep in mind that 2GB of memory (or more), will make Win98 unstable on this platform, without RAM patch from r_lowe (at least, that's my experience with GeForce GPUs with 512MB of VRAM and moded drivers, I didn't tested how 256MB VRAM GF cards react with no RAM patch).
Also, before doing X58 I ran a simple Win98 install test with EP45-UD3P board (with hacked installer), and it worked fine.
I am using Marvell controller in IDE mode for Windows HDD and USB ports in 1.x mode (usb legacy is enabled).
Didn't tested what Intel ICH10R SATA contoller does in Win98 (it's disabled in BIOS), and same goes to other devices/functions that are not needed.
You can see at the end of my video how many devices are simply disabled (or not working), in '98s device manager.
PS. I also switched off connected PCI-e devices by disabling PCI to PCI-e bridges as well (hardcore version of resource conflict management 😀).