I have a Diamond 4870, and I flashed it to the asus overclocked bios. I did a fair bit of research on many of the more extreme overclocking forums. Here is what I found out. If there is enough interest, I can write up a "How to" post for the best chances, at least from what I researched.
1. Bios of the 4870 limits the ccc overdrive. The factory overclocked versions give you a higher range of clocks. So flashing to a factory overclock, will let you set your over drive higher.
2. 4870's use ddr5, and this changed the default bios size. make sure you have the correct bios, for your card. 4850 and 4870 bios' are not compatible because of this change in size.
3. The newer cards, especially the factory overclocks, have better fan settings in the bios, so you do not have to edit the ccc profile. Different cards have different fan settings.
4. All of the 512meg cards are reference design, so any 4870 512meg bios will "effectively" work on any brand.
5. Using winflash seems to have 1/3 chance of bricking your 4870, better to create a boot disk/usb key to windows 98 and use atiflash command line.
6. Make sure you wait for the comfirmation that the flash is completed before rebooting. If it does not confirm a successful flash, flash it again before you reboot.
7. if you Brick your card, W1zzard has a posted a FAQ on how to fix it.
8. Like with any overclocking, different cards will produce different results. Test your card for artifacts with atitool. Let it run for 10-15 minutes straight to ensure there are no artifacts. Some people will suggest longer run times.
9. the 4870 seems to give a wide span of differing results on overclocking. For instance, my core is stable at 800/1050 not artifacts in atitool. average seems to be anywhere between 810-830. Most factory overclock bios default at 815. it will boot to windows fine, but if your card can not handle it in games it will lock up. test it before gaming. I had to underclock that bios to 800 core.
10. If you plan to flash your card, make sure you are ready to use it as a paper weight My heart was pumping rather quickly as i waited for the confirmation that the flash was successful.