1) Download this: http://www.ldb4.com/7800BIOS.ISO
2) If you don't already have a CD burning program download this: http://static.slysoft.com/SetupCloneCD.exe
3) Put in a blank CD-R or CD-RW media.
4) Install and run CloneCD, click the button that says Write from ImageFile, select 7800BIOS.ISO and continue to burn the CD.
5) Reboot, hold F2, select boot from CD drive.
6) Hit enter until you are given the R:\ prompt
7) Type dir /w to see a list of the BIOS files on the CD. They'll look like 425C1200.ROM etc. You'll want to start off with something moderate like 425C1200.ROM before moving up to higher speeds.
8) Type nvflash (name of bios file.rom) and hit Y. For example type 'nvflash 425c1200.rom' without quotes to flash your BIOS to 425MHz core clock and 600MHz memory clock (DDR doubles that 600MHz into 1200MHz effective speed -- just a bit of technical info for ya).
9) Reboot (hit Ctrl-Alt-Delete), run ATItool
and do an artifact scan for 5-10 minutes or so and make sure it doesn't get any artifacts. It should say 0 artifacts in xx:xx minutes which should be the same length of time since you began the test. If you get artifacts the two times will be different and it will also briefly say something about 'delta of xx' meaning it detected artifacts. If you get no artifacts you can safely go up another clock speed so reboot and repeat steps 5-9 until you find a comfortable overclock for your machine. Once you get artifacts in ATItool, back off 5-10Mhz on the core clock and use that.
If you want to go back to the original (stock) clocks, type nvflash original.rom at the R:\ prompt.