From what I know, both the 4850 and 4870 are really solid cards, and should both run WoW a hundred percent smooth in max graphics.
When that's said, 4870 is quite a significant increase in performance, so if you can choose freely, I'd go with the 4870. If you're ever going to see the light and run a bit heavier games, you're gonna be happy you spent that extra buck.
Benchmarks compared to each other here:
http://www.tomshardware.com/charts/gami ... .html?prod[2114]=on&prod[2116]=on
As to the brand, I'd definitely not see Asus as my only choice, there are som very good 4870'es out there from other manufactorers. I couldn't dig out the article, but the Sapphire Toxic Edition is no bad choice:
http://www.tomshardware.com/reviews/rad ... 096-3.htmlIt's silent, it's cool, it delivers what it's supposed to, and it's fairly priced. Just don't go with the Club 3D overclocked-edition, my friend is having stability issues with that one.