Personally, my primary desktop has a reference design Radeon HD6870 in it still. Has been nearly trouble free. I would try a clean Windows install n' all that first if you haven't though. You could have some crapware or malware running in the background or some other odd thing bleeding your performance (unless it's -just- XI).
Neither XI or Diablo III are particularly demanding. Tom's Harware did a Diablo III performance comparison here that might be nice if you do elect to get a new card.
XI Wiki



