i currently have a 4980 OC to 900/1000 and i've been in all the Aion beta. For the most part of the game, 90% of the time it runs it perfectly fine. I have all the settings on max at 1650x1080 but inside cities with a lot of ppl or during raids/rifting with massive people, you can see it slow down but nothing that would affect gameplay too drastically. But the FPS does drop.
A 9800/8800gt will run the game, but i'm sure the FPS would drop a lot more in those intensive moments on max settings than a card like a 4890 or gtx275.
My housemate runs a 4870, and it also runs the game just fine. I would advise you to get a 4870 instead and save a bit of money over the 4890.