I'm assuming that the garbled screen occurs as soon as you fire up the pc,and that you don't see the computers BIOS boot up sequence.
If you backed up your original graphics card Bios (as you should have done) then you will need to flash the card again with the original BIOS.
You will have to do this "blind" if the graphics card does not function....ie boot from the floppy and type in the flash instructions without being able to see what is being typed.
If you dont have the original BIOS backed up the you will need to search the card manufacturers site for the correct one for your card.
If reflashing back to original fails you will have to accept you have broken the card and get a new one.
Incidentally you don't need to flash the BIOS to run the overclock on the 9550 as there are some softmodded drivers available and ATITool click here will unlock it.I believe Omega Drivers have a soft-mod option and are good modded drivers too click here