hdcp can be a pain...i agree with boxzor and hixbot. Its your most likely culprit. Possible its a driver issue (had similar experience with 3870x2 running on the exact same model TV which you speak) though i was able to get it working after about twenty minutes of tinkering. (ie flashing bioses, updating drivers...etc)
an 8800gt should be hdcp complient (question is which version and what version does your TV run). If your TV has a newer spec, your basically screwed tell u upgrade.