Network Transfer Rate on SMB/FTP

We're trying to optimise our network transfer rate from the client PCs to the file server, but having no luck.

If I run an iperf test between a client PC (Windows XP) and our server (Linux) with a large TCP window size (the default is 8kb) I can get 60 Mbps. But when I run an SMB transfer speed test all I get is around 35 Mbps. When I run an FTP transfer speed test all I get is around 32 Mbps.

I thought this was to do with the TCP window size, but I have now increased that to 256960 and it made no difference at all.

Any ideas what I'm doing wrong? Or is 35 Mbps all I should expect?

Cheers,
Jack