Interesting problem, my 32 GB phone will not sync past 2.42 GB available space. If I try to sync even one item say a 100MB playlist iTunes reports that I need 2.42GB + what ever the size of the item is and then states not enough space available. for example I synce 1 play list and iTunes reports not enough space you need 2.56GB of space and only 2.42 is available. MAKES NO SENSE!
I had this happen as well. I can't remember exactly what I did to fix the issue, but I remember un-syncing everything and then adding things back. It took a little time messing with it. Sorry I do not have anything better.