I would start with ATItool, it's pretty easy to use, and it has a "find max core/mem clocks" option that let's it automatically overclock your card in a safe way, you should give it a try. Then you can manualy increase your core and mem speeds by 5mhz (or lower) and test it then increase again until you hit a level where you start seeing artifacts on the screen in 3D applications, then revert back to a lower mhz and start testing your video card at this point, use 3DMark, or play Doom 3 or FarCry for 15-30minutes, if everything's stable, that means you've hit your max speed for your videocard, and you'll be safe (hopefully) at this point.
9800Pro normally max between 400-440Mhz core with stock cooling, but using an aftermarket cooler (IE VGA Silencer) is recommended when OCing because they become really hot after OCing.
I don't know where to find overclocking guides, but you might find some at Rojakpot, not sure though.