rivatuner or i prefer Msi afterburner. and yes keep it under 85C and you should be fine. i made it to 1040mhz Core with my 5770. but you will probably need to increase the voltage to get it that high. dont increase it though till you have to.
i would let furmark run a bit longer than that. maybe 20min. to make sure its stable.
overclocking in general you wont see a huge difference, but the thing is its free performance gains, so why not?