The best tool for this is the command line tool "Rsync". If you need a GUI, try Unison or LuckyBackup, which are based around the concept of Rsync. Unison and LuckyBackup are both in the repositories.
An example of rsync might be :
rsync -vxtr /home/scaine/Pictures/
...which would synchronise my Pictures directory with OtherUser's Pictures directory. The options I've specified will verify any copies, preserve timestamps/permissions during copy and will traverse subdirectories where found. Do "man rsync" for more options.
As for connecting your PCs together - either put them on a switch/hub or use the crossover cable then share the directories via samba.
Some good points to make if the aim is to "mirror" the two PCs.
This command will only "add" files from source to destination. If you need to delete, then you'll need the -delete option in there too.
On investigation, it appears that Rsync does not handle mirroring particularly well. See this to realise that you will probably find older files overwriting younger files during this update! Worse, even if you get the -update option to work and prevent this behaviour, it won't "update" the older with the younger file - it will just "skip" it. Hardly ideal.
Finally, to perform two way sync, you'll need to run the same command but reverse the source/dest. Of course, this is of limited use now, since your first command (assuming you used that -delete option) will have deleted any file on the destination that didn't exist on the source.
Unison handles the two-way sync quite well, from memory.
If you're seriously trying to keep multiple edits in sync, then a Dropbox solution is the best way forward. If you can take another look at either UbuntuOne or Dropbox, symlinking might be a solution - only symlink your documents folder, say, while keeping Pictures and Music off One/Dropbox and using this rsync/unison solution for the big stuff?