All good cards, the 4850 will start to lose steam at that resolution, but it will run, drop some AA or something. I tend to get 60-30fps in WotLK, maxed settings 4xAA. 4850 isn't that much worse than my machine, and its the new shadowing which kills a lot of cards, also fast CPU helps as MMOs in general eat CPU runtime.
Look at some of the models with better stock coolers as some of them run slightly quieter and this is a big issue for some.
I noticed on average a 30-50% improvement going from a 8800GTS512 to a 4870 512 game dependant (I also game at 1920x1200). The higher mem cards will result in smoother gameplay at high res but arent vital if budget is am issue for you. Both cards have improved with drivers, and Im actually finding the HDMI output VERY useful, but thats a limited case scenario, although if I'd been buying now I also would have gone for a 1GB card for future games, but hell, I got this card for £170 months ago when this wasnt an available/cheap option and Im very happy. The additional texture memory aids high res play by allowing smoother texture caching and some improvements to minimum framerate and AA performance. As you're going from the lower texture memory GTS to a 4850 at higher res, I'd imagine you will also get a 30-50% average framerate rise. Hell I got better FPS in Crysis and we know how much that favours NV.
If you go for a 260, try and get a 216 model as they're not much more expensive but are typically 5-10% faster
I also run TF2 Maxed settings, HDR, motion blur, 4 or 8x AA (I forget which) and its 60fps+ vysnc'ed no issue 95% the time, which is great for smooth, good looking gameplay, and this is on a 512mb model, the 1gb is even more friendly for this sort of stuff!
