It means ATI does not support Diablo 3 for now,
However i think you will be able to play it, but the game experience will be not as good as expected.
Get an Nvidia,
Updates once or twices a year, but they work fine, and they even update older cards like my 8800gt ultra and 9800gtx. You should go for an 560gtx, works perfect
. O and best pick them from ASUS, as there are really caring about their costumers.