I played a bit of the open beta on a 4870 (1GB), and had no problems. I wonder if it's due to the VRAM?
As for the game, it seemed polished enough, but unfortunately I didn't have enough time to get into it (only got to about lvl 12). From what I saw, the class/talent structure was interesting. The Rift stuff just reminded me of the public quests in Warhammer. Seemed a bit bland.