I was able to crunch the bigger collatz task with my 560 Ti 2win, no problem.

http://boinc.thesonntags.com/collatz...ltid=136534708

It is on a machine that has the 2win and a 590 (4 GPUs total). Win7 64 (all updates), BOINC 7.0.42, 314.07 driver (boinc shows it at 314.7 in the event log).