The following table highlights the minimum requirements of Windows SharePoint Services (WSS) 3.0 and SharePoint Foundation (SPF) 2010 for preliminary upgrade planning for SPF2010. Since SPF2010 is the underlying infrastructure for SharePoint Server (SP) 2010, information presented here is applicable to upgrade planning for SP2010 as well.
Notice that the following scenarios are not supported in upgrading to SPF2010:
Upgrade from a farm running WSS 3.0 earlier than SP2
Direct upgrade from a farm running WSS 2.0/SharePoint Server (SPS) 2003. In this case, one must go from WSS 2.0/SPS2003 to WSS 3.0/MOSS2007 before going to SPF2010/SP2010.
For production deployment, please do reference the links in Official Requirements of the table to get the latest information.
SharePoint Foundation 2010
64-bit, four cores
4 GB for developer or evaluation use
8 GB for single server and multiple server farm installation for production use
For large deployments, see the "Estimate memory requirements" section in Storage and SQL Server capacity planning and configuration (SharePoint Server 2010).
3 GB for installation
80 GB for system drive
For production use, you need additional free disk space for day-to-day operations. Maintain twice as much free space as you have RAM for production environments. For more information, see Capacity management and sizing for SharePoint Server 2010.
Windows Server 2003 (Standard, Enterprise, Datacenter and Web editions)
Windows Server 2008 (as of WSS 3.0 SP1)
64-bit edition of Windows Server 2008 Standard with SP2
SQL 2000 SP4
SQL 2005 SP2
SQL 2008 (as of WSS 3.0 SP1)
64-bit SQL 2005 with Service Pack 3 (SP3) with Cumulative update package 3 for SQL Server 2005 Service Pack 3
64-bit SQL 2008 with Service Pack 1 (SP1) and Cumulative Update 2 installed with Cumulative update package 2 for SQL Server 2008 Service Pack 1
http://technet.microsoft.com/en-us/library/cc288751(office.14).aspx including software prerequisites
WS2008 R2 Trial, SQL2008 R2 Trial, SPF2010, SP2010 Trial
(A cross-posting from SharePoint Experts Blog)