I leave my media center PC in 1080P 60hz all the time (23hz when watching blu ray or MKVs).
Does this impact what I am watching via my Ceton card for live TV which is I believe is broadcast in 1080i at best (this is the best a set top cable box can do)? Should I be switching to 1080i for live TV and 1080p for blu ray and blu ray rip MKVs etc?
Leave the resolution to the highest quality your display's NATIVE panel will do, always. The difference is deinterlacing the 1080i which is done in the PC easily... any 720p stations will be upscaled in the PC to 1080p as well. The short answer: NO