@Nearly-Right
The cards you mention (realtek 8139 and SB128) are not supported although there is a hack to make the Soundblaster work as a passthrough card.
I have OS4.1 Classic running on my A1200 which has Mediator 1200sx (6 slot)
Its working with my Blizzy PPC + 256mb Ram (Upgraded by Stachu to 330mhz)
I have used it with Radeon 9250 (128mb) and it works great with the 3D Hardware effects

I tested 2 Redeon 9200 256mb and they didn't work though.
I have a Realtek 8029 PCI NIC and I go online without any issues, I also have a SB128 but I didn't try the hack yet.
I tested 2x supported SATA PCI cards and had issues with both, drives are seen and I can partition them but when I save changes the system locks up. I'm going to raise this on the support forums and see if we can get that looked at.
Overall the OS is very responsive and runs well on my A1200 but the issue is the apps/games written for it run slow, Mame, GLQuake, PRDoom all run slow as they are coded for 800mhz Sam's etc...
I think with some apps coded to work with the slower Classic machines it could be a really great OS.
Steve.