I run 500/1000 with my N900 and its great and stable. The temperature gets sometimes high but so far everything looks really great. Even could run 1150 without troubles but was afraid to burn my beloved device.

Some comments on the posts that the phone reboots after chages: If your N900 reboots you may switch off the SmartReflex option. This is a conflict to the settings and lets your phone reboot...at least that is my experience.