The key points: 64-bit only running on 64-bit Windows Server 2008 or 64-bit Windows Server 2008 R2 with a 64-bit SQL Server 2008 or 64-bit SQL Server 2005 to hold the databases.
IE 7 or IE8 as the browser - IE6 is being dropped in July 2010.
(Update: Just noted that IE ONLY is supported by Project Server 2010. Sharepoint Server will be supporting "other" browsers (Firefox and Safari) but it does not appear that cross-browser support will be in Project. See Christophe Fiessinger's Blog )
As most people know, Project Server runs right on top of Sharepoint server, so the same requirements which were announced a couple of days ago for Sharepoint Server apply to Project Server. You can read the details and the QA here: http://blogs.msdn.com/sharepoint/archive/2009/05/07/announcing-sharepoint-server-2010-preliminary-system-requirements.aspx
So far I've seen nothing public about other requirements, but I'd guess that for features such as visual reports that Excel 2003 and Visio 2003 would probably drop off the supported filetype list in this next version and that it would work best with Office 2010.
