Wouldn't it just be easier to buy a slim and then you can keep the old firmware on the fat PS3.
I have a fat PS3 on the newest old firmware I haven't bothered with the PSN or tricking it and I'm thinking about not up grading the firmware on it to keep the other-OS feature in tact.
My parents also have the old 60gig launch PS3 on real old firmware so it also has the other-os feature.
I've been thinking of picking up a slim as well, for the larger HD and yes I do know I can upgrade the HD but I'm not really up for losing out on the other-os feature, so could pick up a slim and just upgrade the HDs in both fat/slim PS3's.
Also if all your PS3's are banned maybe geohot's PS3 is banned as well, seeing as he did the video of CFW and going onto PSN with it.