I updated my 60GB 5G iPod with a 100GB (MK1011GAH) disk that I fluked upon on eBay (they don't seem to be common there). All I had to do was swap the disk out, do a restore from iTunes, then install Rockbox. Of course it's below the 138GB limit, so I didn't have to compile a custom build. The OF seemed to work fine with it, but I only booted into it once out of curiosity, and I didn't have any music in it's database, so I couldn't see if playing anything would actually work. Rockbox of course was fine with it, though I've since transplanted the disk into a Gigabeat S.
At the time I bought the disk, I would have gone for a 160GB, but I couldn't find any. Let me know if you do. I found one place with 120GB's, but they had to order them in, and the price difference from the 100GB was like $100.
EDIT: Reading
shoora's thread, I found that that place
has some, though they don't ship outside the US so I can't get one
