So you want to make the router to force the ACK to be send less frequent?
Samba performs really bad with the delayed Ack... under 100KB/s.2. For some protocols specialized programs (like winscp) may provide a bit of improvement, however that's not an option for Samba and for the rest of the protocols that use TCP (maybe in another Linux box).
Also my Linux PC (with a decent CPU/HDD) when doing a similar FTP copy test was only a bit faster, not reaching the 9 MB/s. So it's not just XP and the FTP that comes with it, but also other OSs have performance issue with TCP.
When I used ubuntu on 2 machines I was capable of reaching over 110MB/s with FTP and samba. I made 2 RAM disks (which is fairly easy in linux) on each pc to make sure that no read/write file delay was bottlenecking the benchmark.
Are you sure both systems had 1GB connections?