Theres some decent info and a few reviews of both these card out there, but not a whole lot since they are fairly new.
There is a great review type article over at [H] for these cards:
[H] Enthusiast - AMD Radeon HD 4800 Series
Some of the issues users have been finding are that this card runs hot. Reportedly idles around 80c, and load temps are around 85c. However, the reason of this is that ATI defaults the fan speed to 10-15 percent, and it doesnt increase while under load. Right now ATI has not released any software or driver updates to control the fan speed, but there is a way to change it. This site has the step by step instructions.
Radeon HD 4850 Heat issues - Guru3D.com Forums
Price points:
The HD4870 can be had for around $310, and the HD4850 can be found for $200 or less.
Personally, the hd4850 seems like a great deal. I've always been partial to Nvidia, but I might have to switch for this desktop I'm in the process of putting together.