I'm running two Titans now. I haven't boxed in months, but when I did I wasn't that impressed. It's not like I could run all 5 windows (1920x1080) at max settings. I stopped ten (and 20 boxing) a while ago so not sure what improvement I would see there.

edit: I am running two monitors at 1920x1080 if you were wondering. 3930, 64GB ram, Samsung 840 pro x2, blah blah