If you want to just flash with the Asus bios unchanged:
1. Use gpu-z (must be latest version or the bios file is not large enough) to take a copy of your existing bios (handy for problems or if you need to put your old bios back on ever)
Winflash in windows will not work with the Asus bios, it comes up with an error so you need to run it in a command prompt as follows:
you have to run it through the command prompt.
Step 1: create a new folder in the root directory of your C drive (so you can easily navigate to it in DOS).
Step 2: copy all the ATI WinFlash files and the new BIOS to this new folder.
Step 3: open command prompt and navigate to your new folder.
Step 4: use the normal ATIFlash command: "atiwinflash -f -p 0 [insert bios name here].rom"
it should take a minute or two to flash. it will tell you to reboot. you'll have to reinstall your video driver. but everything worked for me after that.
WARNING: if you don't get a successful confirmation at the end of the flash, make sure you flash it again before you reboot. you will brick your card if you reboot before a successful flash.
If you don't fancy your card booting up to the default Asus bios speeds of 815/925 then download RBE 1.12 (version 1.11 will not work with 4870 bios) and load up the Asus bios.
Change the clock settings on 0,3,4,5,6 and 9 to whatever you want eg 750Mhz.
should work on all reference cards , ive heard it works on sapphire ones etc , not sure if it will be ok though so use at your own risk , gonna try it on my gainward card. given all the current cards are the same models with different stickers cant see why it wont work tbh.