ATI for Skyrim

Yes you'll be fine. There suggested card for high setting was the gtx260......  

And the HD 4890 also.

@ OP yes it will be fine with that resolution, but you have to consider lowering some options AA, MSAA if supported by the game.

What is mean MSAA? Do you think that this card can show me better graphic than XBOX version, or at least the same?

i would recommend you to upgrade to a HD 5750, MSAA is a technology enhances visuals and the quality of the picture drawn in the screen.
HD 4850 will be fine if you play at medium settings, if you want to play @ high quality get the HD 5750.

Consoles are different than PCs, so yes if you upgrade to a good GPU you'll get better quality.

MSAA = Mulitsampling Anti Aliasing

It is one of many anti aliasing methods that helps improve graphics quality. It's actually quite complicated to explain. Generally speaking though, the higher you set it the more the graphics quality can be improved, but it also lowers performance. Set it too high might result in a slide show if your video card is not powerful enough.

http://n4g.com/news/873610/the-elder-scrolls-5-skyrim-t...

Here is the link that you can see a HD 4850 on MEDIUM list, but they are put 1920x1080 res and say that on lower resolutions game will run better. Also, we know that 1280x1024 is much less than 1920x1080. By this I think that I can get HIGH on my resolution. HD 5750 is on MEDIUM list too, and not much better than HD 4850.

Yes you're right HD 5750 is not that much better but GDDR5 is faster than GDDR3, you'll experience smooth gameplay, high quality textures.

Any way you can try the HD 4850 then decide if you want to upgrade to HD 5750 or higher.