I would think you need *.lpr, *.lpi, *.pas and *.lfm (AKA "the source") if you want to transfer teh project to another machine.
Apply the same directory layout on both machines, this will make things a lot easier.
E.g. I have:
SomePath
|_LazarusProjecten
|_MijnLib
|_SomeProject
|_AnotherProject
on all my machines (actually under svn control).
And only use relative paths, not absolute ones, in your project options.
I can "transfer" each and every project form one machine to another, then build without problems.
Bart