MAKE SURE you go in to the BIOS and set your AGP aperture to 256mb. It will give you a massive performance boost. I did the same when I had my 6800 128mb, and it made it almost exactely the same as my new 256mb 6800gt. If you have a gig of RAM, you really HAVE to have an AGP aperture of 256mb with a 128mb card.
Anyway, to unlock overclocking in the nVidia control panel, open up a txt document and paste THIS into it. Then just save it, rename the file '.reg' instead of '.txt', and run it.
HKEY_LOCAL_MACHINE\SOFTWARE\NVIDIA Corporation\Global\NVTweak]
"NvCplExposeWin2kDualView"=dword:00000001
"NvCplTvLocaleAutoDetect"=dword:00000001
"NvCplAllowForceTVDetection"=dword:00000001
"NvCplEnableActiveApp"=dword:00000001
"DisableApplyColorsAtStartup"=dword:00000000
"NoMenus"=dword:00000000
"OverlayPageState"=dword:00000000
"CinematicPageState"=dword:000000ff
"NvCplThermalSettings"=dword:00000020
"NvCplEnableAppProfilePage"=dword:00000000
"NvCplEnableScreenMenuPage"=dword:00000001
"NvCplCustomResConfig"=dword:00000000
"NvCplEnableAdditionalInfoPage"=dword:00000001
"NvCplDisablePerfQualPage"=hex:00,00,00,00
"NvCplEnableAGPSettingsPage"=dword:00000001
"CoolBits"=dword:ffffffff
"NvCplEnableOverlaySpanControl"=dword:00000001
"TwinViewCapsOverride"=dword:00000001
"NvCplEnableDesktopOverlapPage"=dword:00000001
"NvCplEnableTroubleshootingPage"=dword:00000001
"NvCplDisableRefreshRatePage"=dword:00000000
"NvCplDisableRotateAngles"=dword:00000000
"NoPages"=dword:00000000
"NvCplEnableClearViewPage"=dword:00000001
"NvCplEnableFanControl"=dword:00000003
"NvCplEnableFrameSyncPage"=dword:00000001
"NvCplEnableHardwarePage"=dword:00000001
"NvCplEnablenForcePage"=dword:00000001
"NvCplEnablePwrMzrOnDesktopMenu"=dword:00000001
"NvCplEnableRotationColorSettings"=dword:00000001
"NvCplNvThermPage"=dword:00000001
"PowermizerPreferences"=dword:00000000
"QTwkEnableModeChangeWarning"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\NVIDIA Corporation\Global\OpenGL]
[HKEY_LOCAL_MACHINE\SOFTWARE\NVIDIA Corporation\Global\OpenGL\Debug]
"ShaderObjects"=dword:00000001
After that right click on your desktop, go to properties, settings, advanced, 6600gt, and you'll see 'clock frequency settings' has appeared as an option. To find your maximum overclock, raise the CPU core by about 20mhz, and run 3Dmark05. If there is a problem you will see 'artifacts', which is a euphemism for 'f*ck-ups', appearing. If they don't, keep raising the clock in 10mhz increments until you DO get problems, and then bring it back down by 15mhz.
After that, do the same for your memory. As long as you don't go whacking up the speeds by 100mhz, you'll notice errors long before you actually do any damage.