I have ran it at 650 and default voltage without a fan using a Taisol CGK740, it ran cool enough doing average work, as long as I didn't have a 100%CPU load for more then an hour or so it was fine, so I am very sure running it at 500 and 1.1 volts it would run without the fan. Next step try running it without the heatsink.
Somebody stop me I have gone crazy.