I recently upgraded my HD 4850 card to a HD 5850 card.
It seems to me though that I'm getting much worse performance from it. I've only played World of Warcraft with it thus far but for something that was running fine with maxed settings on a 4850 card I was dumbfounded as to why it was underperforming with a better card.
I did the correct uninstall of the old drivers (safe mode reboot with driver sweeper, etc.) and installed the new drivers without a problem. I can't figure out why the performance is worse though. I scoured the website and only found one similar case but it doesn't seem to relate to my issue. I'm running the 10.5 catalyst drivers on Windows Vista 64bit.
As zipzoomflyhigh stated, the ATI 5850 should be night and day (though may not notice with a game like WoW).
The ATI 4850 is a great card. Consider though that the 4870 (next step up) is about equal to the ATi 5770. And it takes two 5770's to compare to the ATi 5850. So it's a large performance jump.
Anyhow, I would definitely try to benchmark your new video card with other games. WoW isn't really the best game to be testing graphics performance anyhow, as WoW is severely limited by CPU (because of plugins & such) and internet latencies.
A synthetic benchmark like 3dMark06 would give you a much better comparison between the two since it is capable of truly testing over all GPU performance. Then try other games and use something like Fraps (free version) to visually monitor FPS and see if there's a change.
Also, keep in mind, that depending on the resolution you're playing the game at, you may not even see much performance difference. If your CPU is what's already limiting over all performance in certain games, the video card upgrade may not show you any real gains. For example, WoW doesn't really utilize a Quad Core CPU, so even having a great CPU isn't going to change up WoW performance much. That game would probably see more gains with a really fast dual core than a lower speed quad.
I ran 3DMark06 to benchmark my comp. Going back to the WoW thing, what I can't understand is that the difference between running the game on a Friday at 3:00, exiting and installing a new graphics card correctly and running it at 3:30 on Friday why there is a significant loss of framerate. That is what is absolutely confounding me at this moment.
Would it be worth trying out the 10.4 drivers over the 10.5 drivers?
I did these things, ran WoW and that seems to have fixed the problem. It looks like it's an issue with WoW not being recognized as a 3D application.
I appreciate all the help and comments and suggestions that were given. Now it looks like I'll make a final post on Blizz's Tech Support site to see if they have anything planned to fix it and send something to ATI.