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You have a very fast card as it is, and If you dont have very good cooling pushing your card too hard may damage it. I had my 9700 Pro overclocked but then I brought it back to stock speeds because I realized that my games run fine, and the ram was getting quite hot !!
Eventually I want to get some ramsinks and maybe water cool the gpu but thats a long ways off.
I dont think and extra 60mhz will give you outsanding performance gains.
Keep it there. My 9800XT comes stock @ 411 / 361 (or something to that effect).
It's plenty for any modern games. Throw some money aside and wait till the next gen games come out that will require a bit more speed. Then you may actually notice a difference outside benchmarks. Because, honestly, I can't tell
But, if you wish, a lot of people recommend the Arctic-Cooling.com VGA Silencer for 9800Pro chips (not available for the XT, but are now available for the All In Wonder) and overclocking. It costs $21.99 normally.
Another solution is modding a CPU heatsink+fan inplace of the stock one. There's a link on HardOCP's from page, about 1/3rd the way down.
<edit>On second thought, install Lock On: Modern Air Combat, that'll be enough to bring a government super-computer to it's knees. Hehe, man that thing whips on my system like an abusive father!
Speaking it terms of percentages, you should expect to see more than 10% performance increase. Realisticly, you won't notice any difference at all in games. Benchmarks, maybe. I have a 9800 Pro watercooled, and last night I pushed in to 495/380 and it ran solid. I had my APG bus at 70MHz, and my APG voltage at 1.6v. I scored significantly higher in Aquamark, but I didn't noticed much "real" performance increase in games. It rocks at default speed.