I'd leave it altogether if I was you, mate. I OC'd my 4850, the temps were sky high. Eventually my PC started to shut down during gaming due to overheating of the GPU.
So, I got an aftermarket cooler. It wasn't too expensive, just below €20. I came back home, started to undo the screws on the stock cooler, my screwdriver slipped and damaged the card... badly. I killed it.
It's simply not worth it.
The good thing is that I bought a second-hand 4870 three days later.
Not planning on OCing this one, my brand new aftermarket cooler lays on the shelf gathering dust...