samba - Oct 2010 - Slow Samba over VPN

Hello.

I have Samba on Ibex running over VPN, and am having problems with speed 
reading and writing from XP and 7 boxes. It takes, for example, 16 
minutes to transfer four files for a total of 1.2MB.

I assume that this problem is due to SMB block size and the latency I'm 
dealing with, always between 200 and 500ms.

How can I properly configure client and server to negotiate a suitable 
block size / tcp window to reduce the number of transactions and up the 
speed?

Thanks,
Colin