I have a Sony Vaio SZ90 with a nVidia Go 7400. I have tried to use the nVidia programmes to overclock the chip but although the programme reports that the change was successful upon verification the values are still set to Sony's setting.
I think that the custom Sony supplied BIOS has disabled this ability.
I have tried to change with RivaTuner (WinXP) and the nvidea-settings (Linux) utililies. (RIva crashes with a blue screen of death / driver error) and nvidea behaves as I meantoned in the first paragraph.
you could try atitool (yes it sometimes works with nvidia too) or PowerStrip, but generally I'd advice against overclocking a mobile gpu - usually the cooling is barely enough to keep it cool at stock settings, but if you are careful I guess it won't hurt to try.
Just remembered - there's an excellent tool for controlling laptop hardware: Notebook Hardware Control or NHC - you really need to get that one - it's free and might give you control over your CPU speed and voltages. I use it to underclock the laptop in my livingroom, and when lowering the voltages a bit the CPU is much cooler and the fan never starts.