I have nearly 1GB of RAM on my 4000T. The new version of OS4.1 classic will address ZIII RAM cards as system RAM on systems with ZorRAM and BigRAM+ cards soon if not already. My CSPPC has been upgraded to a 420MHZ clocked one and I have a Rev 6 060 running at 75Mhz currently. I dont like OS4 on this machine mostly because of the previous RAM limitations, but it does run very well even at that. It completly screams on 3.9 and I even have mosly true color icons from the OS4.1 set that Ken did awhile back.
Here is my system conig:
CSPPC with 128MB
2X BigRAM+ boards at 512MB
16MB on 4000T
Radeon 9250 with 256MB, using 200 as system RAM through mediator
The key is using your CSPPC SCSI with a, ACARD IDE to SCSI adapter on it and using a fast SSD drive. Its amazing.
I also have kicked everything into fastram I can after boot up since I have so much as far as Amiga RAM goes.
Magnetic has seen my system run and can tell you how fast it feels too 
You might want to ask Karlos if he'd give you his 3.9 setup. The whole OS runs from a RAD: in Fastram. So fast you wouldn't believe it's 25 year old hardware.
