You can but not with the Nvidia driver's control panel.
Download Rivatuner (www.guru3d.com) and within Rivatuner there is a mini registry editor and you can disable the clock test. Then you can OC with Rivatuner till the card blows up if you feel like it. But the card won't be all that stable over the tested speeds, but who knows.
Edit: Within the mini registry editor ("Power User" tab) go under "Nvidia \ Overclocking" and click on "Disable internal Clock test" then add a "1" in the blank on the right.
Then go to "Global \ Overclocking" and Do the same thing for "DisableClocktest"
Also ATI Tool has a good artifacts scanner thing and you can use the scanner even if you have an Nvidia card. This will help you determine when the memory speeds are getting to high. You can get ATI Tool off Guru3d as well.
No problem, you just gotta overclock a few mhz at a time and after each bump up check for artifacts with ATI Tool to monitor the memory and use 3D Mark 03 or something intensive to check the GPU for stability.