You can easily joing Windows 7 to a Small Business Server 2003 network, by simply going to the Computer Properties Box and Joining the SBS 2003 Domain. If however you also use the ConnectComputer Wizard that is available in SBS 2003 to joing a Windows 7 client computer, you will need to install some hotfixes depending on the version of SBS 2003 you are running. Check out Microsoft Hotfixes for this. Its a KB article KB 926505.
Oh and yes, you can connect a Windows 7 64bit Client to an SBS 2003 Domain.
Hope this helps, let me know if I can help anymore.