This is taken from Ultimate3D site - ultimate3d.quakecity.net
Here are the easy steps to overclock your already fast video card.
Go in to your Windows directory and then double click on Regedit.exe
You Registry Editor will open up . Click on HKEY_LOCAL_MACHINE. Scroll down to 'System' and click on that you'll open the CurrentControSet directory. Now click on the 'Services' folder. Scroll down till you see the 'Display' folder. Now click on it and you're in.
Within the 'Display' folder you'll probably have several 000X type folders. These are simply your old/previous 2D/3D card's registry settings. Leave the old ones as they are and simply locate the one that is for your Voodoo3 card. It's easy to spot, as you'll see something like "3Dfx Voodoo3" next to the DriverDesc.
Now click again and you'll open five more folders. Simply open the 'Default' folder and add a NEW STRING VALUE. Name it "Grxclock".
Modify the Grxclock value to whatever you fancy trying your luck at. If you have a 143MHz board and want to try 150MHz then simply try put in "150".
Don't go crazy and set really high clock speeds! 300MHz might sound great but bare in mind the Voodoo3 is NOT a Celeron 300MHz. I suggest increments of about +5MHz.
I call it "No-Pants Wonderday," but it turns out the police just call it "Thursday." Go figure.