Both chips are pretty much badass no matter what, haha. 4890 may be slightly more powerful but it and the 5770 are still going to display some beautiful graphics. I'm not sure what you had before building this computer, but I had a x1950. While that was good back in the day, it's now starting to only handle games on medium and is artifacting like a mother. Can't go wrong with any of these new chips--it's all going to be a big upgrade
Now all we have to do is wait for the card to be delivered!