Stu, I think I may have misinterpreted you.
The Xbox is a powerful media streamer straight away without the need for FTP or anything like that. It can stream all of your music, videos, and pictures from any computer in your house using it's built-in SMB share. It pulls the data from the PCs (rather than the push through SMB or FTP that you are talking about).
You should be able to browse your network under the "SMB shares" option in XBMC. Alternatively, you can create shortcuts in XBMC to point directly to your file shares with the necessary path/login information to make it work. Take a look at http://www.xboxmediacenter.com
for more info.
To summarize, enable file sharing on your media folders on your PC/laptop. Access these shares through the "SMB shares" section on XBMC (they should show up automatically). If not, then you will need to manually configure SMB shortcuts on XBMC to get at your media. (I have shortcuts setup anyway, because it's less button presses to get at what I want)
Once you have streaming setup properly, it's as simple as downloading to your PC, and pressing play on your Xbox.