I actually use rivatuner and atitool at the same time. i do the overclocking with rivatuner and the stability testing with either atitool:
techPowerUp! :: Downloads
or FurMark, it's pretty crazy and I find it stresses my cards even more than ATITool
FurMark: Graphics Card Stability and Stress Test, OpenGL Benchmark and GPU Temperature | oZone3D.Net
Anyways, back to RivaTuner, when you open it up under the "Main" tab there is the "ForceWare detected.................Customize" drop down menu if you hit the little triangle. When opened you should see a little graphics card icon, click on it.
Here is where you access your can speeds and clocks. I usually do one at a time, I unlock the core and shaders and go from there. When artifacts start showing up in my stressing program I back off and let it sit for a few hours. If that passes then i go to shaders and then memory...bumping it up 5 or 10Mhz at a time. Do a quick look around to see what others are getting on their 9800OCs and then as you get close shrink the increments of increase.
Once you think you've found the max for all three then start up your stressing program and run it over night. If she passes then you're golden. Make sure to monitor your temps.
Further down the "Overclocking" tab is the "startup settings", check that off if you want to run your OC 24/7. The next tab is "Fan", click there and set the option to direct control and crank up the fan if you don't care about noise. I run mine at 100% all the time, my office sounds like a wind tunnel with all the computers running so who cares if another one is added to the mix. It also has the option further down to apply this custom setting at startup.
Enjoy, hope that helps.