SharePoint Server 2010 will be 64-bit only and require 64-bit Windows Server 2008 or 64-bit Windows Server 2008 R2 to run. It also will require 64-bit SQL Server 2008 or 64-bit SQL Server 2005.
According to the SharePoint Team Blog SharePoint Server 2010 won’t support Internet Explorer 6. SharePoint 2010 will be “targeting standards based browsers (XHTML 1.0 compliant) including Internet Explorer 7, Internet Explorer 8 and Firefox 3.x. running on Windows Operating Systems. In addition we’re planning on an increased level of compatibility with Firefox 3.x and Safari 3.x on non-Windows Operating Systems”