Copied from GeForce2 MX flashing thread at bbs.xlr8yourmac.com . Open Firmware commands to correct scrambled video on PCI GeForce2 MX after flashing:

Boot into open firmware by using cmd-option-o-f. This will bring you to a command prompt, however, you may not be able to read it because of the screwed up GeForce. However, you can type blind and it will still work.
Step 4: type the following commands at the open firmware prompt:
> dev pci
> dev NVDA,NVMac
> ddc2-set-start
> setup-clut
> boot
The system will start, but it will probably still be scrambled. If it is, push reset and zap the PRAM with cmd-option-p-r. After it is zapped, you should have a beautiful mac smiley face and some fast quake 3 action!