Although it is only good for 430W, that Crappermaster PSU is not their worst: http://www.hardwaresecrets.com/article/550
and a 430W PSU should be able to run a HD7770 without problems.
Your problem does look like thermal throttling, and it could be of the CPU as well. Do you have good airflow through your case? Make sure your fans aren't fighting each other so you do have flow through your case.
What graphics card did you have before? Did you clean off the old drivers before you installed the new card?