Here's a simple question for you - how much memory can SQL Server 2005 support?

Well, the answer isn't as simple as it might at first seem because, in some cases, the upper limit is bounded only by the OS and hardware.  So, the first rule of thumb is this:

    - SQL Server 2005 Express - 1gb
    - SQL Server 2005 Workgroup - 3gb
    - SQL Server 2005 Standard and Better - OS Maximum
    (As described in http://www.microsoft.com/sql/prodinfo/features/compare-features.mspx)

Taking the shortcut of "OS Maximum" is very useful from a documentation standpoint because future iterations of the OS and hardware are likely to have ever-increasing upper bounds.  However, here are some useful rules of thumb to keep in mind (remember that these only apply currently extant platforms):

    - Windows 2003 R1 Enterprise Edition 32-bit - 32gb 
    - Windows 2004 R2 Enterprise Edition  - 64gb
    - W2003 EE 64-bit - 1tb

If you're looking for the background information on each of these OS and hardware platforms check these references out:

SQL 2005 Editions:
http://www.microsoft.com/sql/prodinfo/features/compare-features.mspx
SQL 2000 Editions:
http://msdn2.microsoft.com/en-us/library/ms178067.aspx

Windows Server 2003 32-bit and Itanium Info (with mostly complete info
for max memory):
http://www.microsoft.com/windowsserver2003/evaluation/sysreqs/default.mspx

x64 Editions:
http://www.microsoft.com/windowsserver2003/64bit/x64/standard.mspx
http://www.microsoft.com/windowsserver2003/64bit/x64/enterprise.mspx
http://www.microsoft.com/windowsserver2003/64bit/x64/datacenter.mspx

All Editions - Features Only (no specs):
http://www.microsoft.com/windowsserver2003/evaluation/features/comparefeatures.mspx

Cheers!

-Kevin

(Thanks to SQL Server MVPs Erland Sommarskog, David Gugick, Ron Talmage, Geoff Hiten, Linchi Shea, and Chuck Boyce as well as Microsoft's Peter Byrne for this tip!)