First: how do you like the sound of your 5.5g? I really enjoyed mine. I bought the classic when it came out (the first 160gb model they released, so it's thicker) and really don't enjoy the sound as much as I did on the 5.5g. Also, all the "bells and whistles" that came with my classic resulted in a less responsive interface (perhaps they've sorted this with a faster processor since the first (6g) Classic. The 5.5g is MUCH snappier at browsing music for me.)
As such, I've hung on to my 80gb 5.5g and plan on eventually doing exactly that: upgrading to 240gb and replacing the battery. SQ I enjoy and enough capacity to tide me over for years to come.
Never really thought about rockboxing it tho, interesting.