SQL SP4 has now been released
http://www.microsoft.com/sql/downloads/2000/sp4.asp
I've been running the beta on my development box with no problems, although I personally do not have a compelling reason to upgrade from SP3a.
One new piece of functionality is the I/O monitoring built into the product
http://msdn.microsoft.com/sql/default.aspx?pull=/library/en-us/dnsqldev/html/sqldev_04222005.asp