Wireless throughput across LAN

http://www.oreillyne...throughput.html
describes a single 802.11 transaction well - I wish I had found that earlier. But I would like to consider a more complicated scenario - the simplest home network:
Router set to 802.11g only, with a Desktop and a Netbook each with g capability.
Both computers have Win XP Home SP3 and are set up identically network-wise, same username and password.
The task is to send a 30 MB file from Desktop to Netbook.
Since each packet has to make 2 journeys (from Desktop to Router, then Router to Netbook) does this exactly halve the payload throughput ?

When I use a packet sniffer on Desktop, I see 2 packets with payload of 1,448 bytes for every ACK it gets back. Can you explain this ?

I see about 11 Mbps over the Desktop-Router connection and 1.0 MBps for the file transfer.
Can this be tuned up further ?