Push the core clock / memory separately.
Up your clocks by a little and test until they become unstable, e.g. once you start seeing artifacts etc in your testing.
From there you can either back off by 10-15mhz to determine stability at your current voltage and/or up the volts to accommodate higher clocks.
Core clock is more important as it will yield better performance gains, so see how far you can get with that first then move to the memory.
Now the memory is going to be a little different; it has error correction so you're going to need to keep an eye out for diminishing returns in performance (even a loss) once you push the memory clocks too high.
1.3v is about the highest you're going to want to put on the card, but make sure you keep an eye on your temps incl. your VRMs which you'll be able to view in GPUz.
A 5850 is within 3-8% of a 5870 clock-for-clock, so it's definitely a nice bump in performance once you push the card.
Good luck and have fun floggin that card