SharePoint Server 2010 will support only 64 - bit. It will require 64 bit Windows Server 2008 or 64 bit Windows Server 2008 R2. In addition to this, it will require 64 bit version of SQL Server 2008 or 64-bit version of SQL Server 2005.
Yes of course you can install SharePoint in client OS also Windows 7 64 bit and for the first time non-server operating systems – are supported for WSS 4.0 and SharePoint Server development.