you didn't apply more volts, your card couldn't do what you asked it to with the volts it is given, increase voltage if you want to overclock that much, i would also be wary of temps when overclocking that far.
um yeah for 5850 you really cant go to far past 800 with out a slight voltage jump
Its reasonably safe (assuming you run the fan faster) to increase the voltage in between 1.2-1.3v (once again assuming you run the fan faster to keep VRMS and Core cool) and that should easily allow you to hit the 1 ghz mark.
Granted not all 5850s are the same so you might get more or less but it seems that most people can hit 1 ghz with 1.2-1.25vs
Most people also say that increasing the volts past 1.3 isn't really helping and that around 1 ghz is the max stable OC
Personally I'm using MSI afterburner with the core at 1 ghz, with 1.25vs, with a custom fan profile that has the fan idling at 35% and going to 100% at 80 to keep everything cool
Did the card crash and you powered off the system and thats it? Or did you try playing games after you over clocked it and its not working?
As the others have said - you cannot overclock the card to destruction unless you add voltage - and even then the card will downclock itself.
As wayne mentioned, overclocking requires voltage and that voltage generates more heat. He runs his fan at 100% to keep things at 80C, I run my fans at 80% to keep things under 70C (not much noise difference) at 850MHz.