Stop me if you've heard this, but... 9800 is a model number for ATi's Mobility Radeon series, not Nvidia. And in answer to your question, yes, it's possible that underclocking will extend battery life, but I don't know how much. At the moment, my video card (Nvidia GeForce GO 6600) is overclocked, and I'm not noticing a drop in battery life, so I wouldn't expect the reverse to occur. And Pentium 4-based notebooks (which I'm assuming you have, based on the thread location) don't generally get very good battery life due to the power consumption of the CPU, so altering the clock speed on the video card would most likely have minimal effect. However, feel free to try! It won't hurt, and you can clock it back up to normal if it doesn't work!