Either of those is plenty powerful for 1280x1024. But you probably can
get a 5770 if you get Powercolor's 512mb version
(it's cheaper than most 1gb 5750s) since, even with 4xAA enabled, you'll never get close to using 512mb of video memory at that resolution.
Now, if you are planning on upgrading your monitor anytime soon, that's another story.