i have a rather old, quite low-powered laptop that i would like to try to use in the following way:
1) grab files from the /home partition on my other laptop thru the wireless
2) takes a list of modified files from after the last backup
3)pasting modified files to the large hard drive attached to the router (it can already be pasted to and everything so thats fine )
4)i would also like it to do a similar thing for certain folders on the windows xp box upstairs.

the way i IDEALLY see it working is user send shutdown signal, and once the hard disk has done a bit of ticking sends them files into the ram of the laptop (only little mind: say 128mb) then this can send the files to the rather slow network storage

sorry for the rambling description, not sure how best to describe it clearly

if it all works out i'll buy myself one f those raspberry pi PC's to do it in place of the laptop (one watt at load: could be on all night)

thank you if you had the patience to read

I'm not sure if this will meet your criteria, but here is what I have done to solve a similar backup situation:

I use rsync across Linux/Windows.

I configured an rsync server on my main system with a separate stanza for each machine that I back up.

On the Linux clients, I just configured the rsync daemon to sync my data to the server.

on Windows clients, I used Deltacopy, which is an rsync client/server for windows.
I set the scheduled task to occur at logoff. It adds a little time to the shutdown sequence, but it works great.
When I first configured it, I let the system complete one full backup, and now it usually only takes a minute or so.