Just get 60 GB with new Jasper motherboard.
20 GB with Falcon motherboard is start phasing out right now.
I have Xbox 360 with Opus motherboard (original first Xbox 360) and 120 GB HDD (upgraded from 20 GB last year), I was think about want buy new Jasper based Xbox 360 and sell my old Xbox 360 and 60 GB to somewhere, swap 60 GB off and put existing 120 GB in later.
Does my 120 GB HDD will work with new Xbox 360 on different console ID? I'm concern that game saves, demo, arcade, others to unable to get work on new console due DRM.