These new cards from ATI are aimed for the gamers who wants high end performance without having to shell out insane amount of money for it. At a MSRP of 200$ for the 4850 and 300$ for the 4870, these cards are dealing a heavy blow to the green giant who has been for a long time using his performance crown to charge a hefty premium for gamers who wants high end FPS.
The 4850 and the 4870 are much alike in terms of features, they have a single GPU based on the 55nm RV770 chipset and both cards have 965 millions transistors, 800 stream processing units and 40 texture units all on a 256-bit bus. The differences between the 2 cards are on clock speeds and memory, the 4850 is a single slot card with GDDR3 memory clocked at 625/1000Mhz (1986Mbps) and the 4870 got a dual slot cooler on it with GDDR5 memory clocked at 750/1800Mhz (3.6Gbps).
The 4850 performs pretty much the same as the 9800 GTX from Nvidia while the 4870 brings the war to the GTX 260 and sometimes outperforms it. Now its up to you to chose which brand you will get for your next video card, but thanks to ATI, gamers on a budget are now getting more for their money.

-Here are some reviews and benchmarks-

http://www.guru3d.com/article/radeon-hd-4870-review--asus/
http://en.expreview.com/2008/06/24/first-r...70-and-hd-4850/
http://techreport.com/articles.x/14990
http://www.pcper.com/article.php?aid=581&type=expert