I just upgraded from a gtx285 to a gtx660ti ftw. When I installed the new card and tried to boot up the monitor says no signal. When I reinstall the old card it boots up fine (this is how I am typing this now). Both cards us 2 6 pin connectors for power and I used the same on both cards. I have a 700 wat psu. What else can I try to troubleshoot?
Make sure the new card is seated all the way and check the power wires. Other than that there is not much you can look at if it wont post. Sounds like the 660 is doa. Do you have another computer you can try it in?