4890 is only 10% faster than 4870. Actually an Oced 4870 can perform as well as stock 4890. Now if you are not OCing I would say save your money and get the HD4870, but if you are planing to OC than 4890 is a beast. Most of the review sites were able to OC the card to 1GHz on the GPU, but then as I said is all relative. do you want to pay extra $50 for play at 57FPS instead at 52FPS. Your eyes wont notice the difference.
Playing at 1920x1080 is smooth under both cards, it all depends how much AA are you using in games.
I think some people want to have the newest baddest card. It's an expensive habit but if they can afford it more power to them.