Thanks to Ramesh Viswanathanfor the tip: Microsoft has released Service Pack 4 for SQL Server 6.5. The Service Pack ups the build number of SQL Server from 6.50.201 to 6.50.281, by the way (Service Pack 3 was 6.50.258). You can download the Service Pack directly the Microsoft FTP site. There are Intel and Alpha versions available.
I installed the Service Pack today on my Windows NT Server system with SQL Server Service Pack 3 previously installed, without incident