Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
You'll need to flash the card with the 4870 ROM, and you'll get maybe a 7% increase in power. It isn't worth it unless you sell your 4870 afterward.

Also, my signature.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.