It won't be harmful to the system, but you could end up with a dead card.... that is something all of us who have Flashed their GPUs know, that the risk that is taken.
You'd need 2 to 3 tools to do it. One would be ATI Flash, One would be WinFlash and RBE.
Also you'd need to save your current BIOS on the GPU to a file that you could access later if you need to reflash it back to normal factory conditions.
You can get those utilities here...http://www.techpowerup.com/downloads/Utilities/BIOS_Flashing/ATI/http://www.techpowerup.com/downloads/Utilities/RBE/
I would suggest going through the steps atleast 3 to 4 times and making sure you have everything in place and then going ahead with this.
Do remember to read up on the process, it's simple but requires your attention.