HD4870 third party cooler

I've heard people getting nice temp drops just by replacing the stock TIM with some quality paste. (and upping the fan speed, of course   )

If it will fit, the Accelero S1 Rev. 2 will tame your heat problems.
A little big but it can keep your card nice and cool.

The Thermaltake DuOrb is suposed to do a decent job of cooling the 4850 and should work with the 4870 but I think it is right at the edge of what it can handle.

Due to packaging constraints, I just orderd a pair of Zalman VF1000's for my 4850's.
They are suposed to do a good job at cooling these cards, nearly (but not quite) matching the Accelero.
If you are going to CrossFire and have space constraints, this is about as good as you can get for now.

EDIT:
Have you tried fixing your fan speed?
A quick googling will find many posts like This one detaling how it is done.
Try setting it to 30 or 40% and see what your temps are like then...

The stock 4870 cooler is actually a pretty good solution, as it really is an efficient design and pumps heat outside the case unlike most of the third party alternatives. Having the hot air building in the case isn't very helpful for CPU or chipset temps.

My 4870 (with stock cooler but 3rd party thermal grease) runs 42C at idle and around 55C peak with the fan fixed at 45% and the largest available overclock (granted, it isn't much). That's pretty chilly for this GPU, and it isn't that loud either.