There are two ways to increase the voltage of your video card. One way is to use the Pencil mod, where by reducing the resistance of a certain resistor by 15ohms to increase voltages by .04v to .06v's. A reduction of 30ohms will net an increase of .08v's to 1.2v's. The resistor number on the 9800 series cards is R1597. Do this by running a 2b pencil from one side to the other, swiping once and measuring with a digital multimeter. Of course, use the digital multimeter to get an original ohms reading before you start using the pencil(graphite) to reduce the ohms on said resistor. Not sure what resistor you would need to perform this on with the card you have. There are extensive guides to this mod, one of which is hosted and authored by W1zzard from this web site. I suggest you find it and read it throughly before implementing and changes.

BTW, the other way to mod it for voltage increases requires soldering skille and electrical pots.

I have sucessfully volt modded every ATI card I have ever owned. And it does allow higher clocks on the core. But beware, higher voltages can shorten the life of ypur card. And too much at one time will kill it. It is paramont to have the proper tools before you start this endeavour. Good luck!