I've got a 4th gen rockboxed and a 5.5th gen (80gb) rockboxed, and yes, if you plan on having your entire collection in flac (which i'm trying to do), you better have at least 40gb. My only beef is the fact that right now with the latest daily build of rockbox for the 5.5 gen model ipods, you either have to choose not many "wow" features, or go with the basic features. So, if you want album art resizing, as well as FF/RWD scrub features, among others, you have to go with an unsupported build, and for me, i have song skipping issues with unsupported builds (that i can tell). Long story short, it's a trade off to get a decent/usable interface for your DAP + FLAC/OGG support. Also, the 5.5gen ipods when rockboxed just seem so much more sluggish in comparison to my 4th gen rockboxed. I would love to just use my 20gig 4th gen, but 20 gig just is too small for a nearly all FLAC/OGG media library. Anyway, I had considered selling my 5.5th gen on ebay and getting a 20gb X5L and upgrading the hd to 80 gig or some such. I may look into just upgrading the 4th gen ipod. Anyway, sorry to hijack and ramble, was mostly just wanting to put my input in regards to rockbox and ipods.
Let's keep this thread going, it can definitely be useful for others.