I vaguely remember it being along the lines of:
128MB RAM (Fast RAM - you only need chip RAM if you wanna use the classic chipset)
AGA is the minimum chipset required, but I'll bet that's painful to use. I dunno what support is like for classic graphics cards...
A supported PPC CPU (these started at 160MHz on the Blizzard PPC I think)
A few GB of hard drive space and an optical drive
ROM / EEPROM? Not necessary. Kickstart is loaded into RAM from the hard drive during boot.
64MB will do although 128MB is very recommended.
AGA isn't the minimum. OS4 will also run on an A3000 with ECS for example.
You don't need a few GB of HD space, it should fit comfortably in a few hundred MB.
My advise would be disabling the os4 themes and going back to plain 3.x theme. Using a gfx card is a very good idea (but I would skip ZorroII ones). If you own a CSPPC you should use SCSI interface. Fit the maximum quantity of ram you can: 128MB on CSPPC, 256MB on Blizzard.
Since scsi is not supported on Blizzard it's advised to send your board to Karlos so he can do a nice scsi driver ;-)
