When I did it, it just ignored the key, so what do you do? It wont work and the new limit is not used.
Well first make sure you are running SP3 of Exchange then you need to apply the following roll up patch:
Exchange 2000 Post-Service Pack 3 (SP3) Rollup Patch 6487.1
The filename is: Exchange2000-KB824282-x86-enu.exe
I applied this and rebooted the server and then the registry key worked, and saved alot of running around!
If you have already put in the temporary key for 17GB and are over that then you must do a defrag of the database and you will have downtime.
See these links for more information:
Exchange 2000 Post-Service Pack 3 (SP3) Rollup Patch 6487.1
http://support.microsoft.com/default.aspx?scid=kb;en-us;824282