I bought a 3870 2 months ago. Very pleased with it but when you read reviews about the 4850 and 4870 it makes you think the 3870 isa slouch! Probably in less than 12 months timne the 4800's will look like the 3870 does now! Being an ATI fan, I'm pleased to see that they are now up with Nvidia.
Here are Toms Hardware conclusions after testing the 4850
AMD Radeon HD 4850 A too-hasty launch, but a card with a red-hot performance/price ratio despite its use of a reworked architecture, now optimized and extremely effective at this price level. Despite its advance in presenting the GeForce 9800 GTX +, Nvidia has been bested for performance/price ratio and at this price level.
Pros Optimized and corrected RV670 architecturePerformance on a par or slightly better than the GeForce 9800 GTX +Antialiasing often performs better than GeForce Cons Higher power use and temperature at idle
It will be interesting to see the results of the 4870 when they test it
I too was always an ati man but have now switched to Nvidia.I'm always surprised how power hungry the ATI cards are compared to their Nvidia equivalents. And it seems that a lot of games seem biased towards Nvidia and configure their software accordingly. Or am i paranoid?